diff --git a/changes/2252.feature.rst b/changes/2252.feature.rst
new file mode 100644
index 0000000000..42b21fd999
--- /dev/null
+++ b/changes/2252.feature.rst
@@ -0,0 +1 @@
+The typing for Toga's API surface was updated to be more precise.
diff --git a/core/src/toga/__init__.py b/core/src/toga/__init__.py
index 09bfa38098..87fe6b4582 100644
--- a/core/src/toga/__init__.py
+++ b/core/src/toga/__init__.py
@@ -1,4 +1,7 @@
+from __future__ import annotations
+
 import warnings
+from pathlib import Path
 
 from .app import App, DocumentApp, DocumentMainWindow, MainWindow
 
@@ -49,7 +52,7 @@ class NotImplementedWarning(RuntimeWarning):
     # single argument (the warning message). Use a factory method to avoid reproducing
     # the message format and the warn invocation.
     @classmethod
-    def warn(self, platform, feature):
+    def warn(cls, platform: str, feature: str) -> None:
         """Raise a warning that a feature isn't implemented on a platform."""
         warnings.warn(NotImplementedWarning(f"[{platform}] Not implemented: {feature}"))
 
@@ -116,7 +119,7 @@ def warn(self, platform, feature):
 ]
 
 
-def _package_version(file, name):
+def _package_version(file: Path | str | None, name: str) -> str:
     try:
         # Read version from SCM metadata
         # This will only exist in a development environment
diff --git a/core/src/toga/app.py b/core/src/toga/app.py
index 5bef2bae82..79e972ca44 100644
--- a/core/src/toga/app.py
+++ b/core/src/toga/app.py
@@ -6,22 +6,14 @@
 import sys
 import warnings
 import webbrowser
-from collections.abc import (
-    Collection,
-    ItemsView,
-    Iterator,
-    KeysView,
-    Mapping,
-    MutableSet,
-    ValuesView,
-)
+from collections.abc import Iterator
 from email.message import Message
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Protocol
+from typing import TYPE_CHECKING, Any, MutableSet, Protocol
 from warnings import warn
 from weakref import WeakValueDictionary
 
-from toga.command import Command, CommandSet
+from toga.command import CommandSet
 from toga.documents import Document
 from toga.handlers import wrapped_handler
 from toga.hardware.camera import Camera
@@ -32,10 +24,10 @@
 from toga.screens import Screen
 from toga.types import Position, Size
 from toga.widgets.base import Widget
-from toga.window import Window
+from toga.window import OnCloseHandler, Window
 
 if TYPE_CHECKING:
-    from toga.icons import IconContent
+    from toga.icons import IconContentT
     from toga.types import PositionT, SizeT
 
 # Make sure deprecation warnings are shown by default
@@ -43,7 +35,7 @@
 
 
 class AppStartupMethod(Protocol):
-    def __call__(self, app: App, **kwargs: Any) -> Widget:
+    def __call__(self, app: App, /, **kwargs: Any) -> Widget:
         """The startup method of the app.
 
         Called during app startup to set the initial main window content.
@@ -53,11 +45,10 @@ def __call__(self, app: App, **kwargs: Any) -> Widget:
             future versions.
         :returns: The widget to use as the main window content.
         """
-        ...
 
 
 class OnExitHandler(Protocol):
-    def __call__(self, app: App, **kwargs: Any) -> bool:
+    def __call__(self, app: App, /, **kwargs: Any) -> bool:
         """A handler to invoke when the app is about to exit.
 
         The return value of this callback controls whether the app is allowed to exit.
@@ -69,21 +60,19 @@ def __call__(self, app: App, **kwargs: Any) -> bool:
         :returns: ``True`` if the app is allowed to exit; ``False`` if the app is not
             allowed to exit.
         """
-        ...
 
 
 class BackgroundTask(Protocol):
-    def __call__(self, app: App, **kwargs: Any) -> None:
+    def __call__(self, app: App, /, **kwargs: Any) -> object:
         """Code that should be executed as a background task.
 
         :param app: The app that is handling the background task.
         :param kwargs: Ensures compatibility with additional arguments introduced in
             future versions.
         """
-        ...
 
 
-class WindowSet(MutableSet):
+class WindowSet(MutableSet[Window]):
     def __init__(self, app: App):
         """A collection of windows managed by an app.
 
@@ -92,7 +81,7 @@ def __init__(self, app: App):
         :attr:`~toga.Window.app` property of the Window.
         """
         self.app = app
-        self.elements = set()
+        self.elements: set[Window] = set()
 
     def add(self, window: Window) -> None:
         if not isinstance(window, Window):
@@ -113,7 +102,7 @@ def discard(self, window: Window) -> None:
     # 2023-10: Backwards compatibility
     ######################################################################
 
-    def __iadd__(self, window: Window) -> None:
+    def __iadd__(self, window: Window) -> WindowSet:
         # The standard set type does not have a += operator.
         warn(
             "Windows are automatically associated with the app; += is not required",
@@ -122,7 +111,7 @@ def __iadd__(self, window: Window) -> None:
         )
         return self
 
-    def __isub__(self, other: Window) -> None:
+    def __isub__(self, other: Window) -> WindowSet:
         # The standard set type does have a -= operator, but it takes sets rather than
         # individual items.
         warn(
@@ -136,10 +125,10 @@ def __isub__(self, other: Window) -> None:
     # End backwards compatibility
     ######################################################################
 
-    def __iter__(self) -> Iterator:
+    def __iter__(self) -> Iterator[Window]:
         return iter(self.elements)
 
-    def __contains__(self, value: Window) -> bool:
+    def __contains__(self, value: object) -> bool:
         return value in self.elements
 
     def __len__(self) -> int:
@@ -155,7 +144,7 @@ class WidgetRegistry:
     # values()) are all proxied to underlying data store. Private methods exist for
     # internal use, but those methods shouldn't be used by end-users.
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args: Any, **kwargs: Any):
         self._registry = WeakValueDictionary(*args, **kwargs)
 
     def __len__(self) -> int:
@@ -171,19 +160,15 @@ def __iter__(self) -> Iterator[Widget]:
         return self.values()
 
     def __repr__(self) -> str:
-        return (
-            "{"
-            + ", ".join(f"{k!r}: {v!r}" for k, v in sorted(self._registry.items()))
-            + "}"
-        )
+        return f"{{{', '.join(f'{k!r}: {v!r}' for k, v in sorted(self._registry.items()))}}}"
 
-    def items(self) -> ItemsView:
+    def items(self) -> Iterator[tuple[str, Widget]]:
         return self._registry.items()
 
-    def keys(self) -> KeysView:
+    def keys(self) -> Iterator[str]:
         return self._registry.keys()
 
-    def values(self) -> ValuesView:
+    def values(self) -> Iterator[Widget]:
         return self._registry.values()
 
     # Private methods for internal use
@@ -213,8 +198,8 @@ def __init__(
         resizable: bool = True,
         minimizable: bool = True,
         content: Widget | None = None,
-        resizeable=None,  # DEPRECATED
-        closeable=None,  # DEPRECATED
+        resizeable: None = None,  # DEPRECATED
+        closeable: None = None,  # DEPRECATED
     ):
         """Create a new main window.
 
@@ -262,7 +247,7 @@ def on_close(self) -> None:
         return None
 
     @on_close.setter
-    def on_close(self, handler: Any):
+    def on_close(self, handler: OnCloseHandler | None) -> None:
         if handler:
             raise ValueError(
                 "Cannot set on_close handler for the main window. "
@@ -288,7 +273,7 @@ def __init__(
         (e.g., due to having unsaved change), override
         :meth:`toga.Document.can_close()`, rather than implementing an on_close handler.
 
-        :param document: The document being managed by this window
+        :param doc: The document being managed by this window
         :param id: The ID of the window.
         :param title: Title for the window. Defaults to the formal name of the app.
         :param position: Position of the window, as a :any:`toga.Position` or tuple of
@@ -318,7 +303,10 @@ def _default_title(self) -> str:
 class App:
     #: The currently running :class:`~toga.App`. Since there can only be one running
     #: Toga app in a process, this is available as a class property via ``toga.App.app``.
-    app: App = None
+    app: App
+    _impl: Any
+    _camera: Camera
+    _location: Location
 
     def __init__(
         self,
@@ -326,15 +314,15 @@ def __init__(
         app_id: str | None = None,
         app_name: str | None = None,
         *,
-        icon: IconContent | None = None,
+        icon: IconContentT | None = None,
         author: str | None = None,
         version: str | None = None,
         home_page: str | None = None,
         description: str | None = None,
         startup: AppStartupMethod | None = None,
         on_exit: OnExitHandler | None = None,
-        id=None,  # DEPRECATED
-        windows=None,  # DEPRECATED
+        id: None = None,  # DEPRECATED
+        windows: None = None,  # DEPRECATED
     ):
         """Create a new App instance.
 
@@ -357,7 +345,7 @@ def __init__(
                For example, an ``app_id`` of ``com.example.my-app`` would yield a
                distribution name of ``my-app``.
             #. As a last resort, the name ``toga``.
-        :param icon: The :any:`icon <IconContent>` for the app. Defaults to
+        :param icon: The :any:`icon <IconContentT>` for the app. Defaults to
             :attr:`toga.Icon.APP_ICON`.
         :param author: The person or organization to be credited as the author of the
             app. If not provided, the metadata key ``Author`` will be used.
@@ -447,7 +435,7 @@ def __init__(
         if app_id:
             self._app_id = app_id
         else:
-            self._app_id = self.metadata.get("App-ID", None)
+            self._app_id = self.metadata.get("App-ID")
         if self._app_id is None:
             raise RuntimeError("Toga application must have an app ID")
 
@@ -485,24 +473,24 @@ def __init__(
 
         self.on_exit = on_exit
 
-        # We need the command set to exist so that startup et al can add commands;
+        # We need the command set to exist so that startup et al. can add commands;
         # but we don't have an impl yet, so we can't set the on_change handler
         self._commands = CommandSet()
 
         self._startup_method = startup
 
-        self._main_window = None
+        self._main_window: MainWindow | None = None
         self._windows = WindowSet(self)
 
-        self._full_screen_windows = None
+        self._full_screen_windows: tuple[Window, ...] | None = None
 
         self._create_impl()
 
         # Now that we have an impl, set the on_change handler for commands
         self.commands.on_change = self._impl.create_menus
 
-    def _create_impl(self):
-        return self.factory.App(interface=self)
+    def _create_impl(self) -> None:
+        self.factory.App(interface=self)
 
     ######################################################################
     # App properties
@@ -547,12 +535,12 @@ def home_page(self) -> str | None:
     def icon(self) -> Icon:
         """The Icon for the app.
 
-        Can be specified as any valid :any:`icon content <IconContent>`.
+        Can be specified as any valid :any:`icon content <IconContentT>`.
         """
         return self._icon
 
     @icon.setter
-    def icon(self, icon_or_name: IconContent) -> None:
+    def icon(self, icon_or_name: IconContentT) -> None:
         if isinstance(icon_or_name, Icon):
             self._icon = icon_or_name
         else:
@@ -631,23 +619,23 @@ def main_loop(self) -> None:
         self._impl.main_loop()
 
     @property
-    def main_window(self) -> MainWindow:
+    def main_window(self) -> MainWindow | None:
         """The main window for the app."""
         return self._main_window
 
     @main_window.setter
-    def main_window(self, window: MainWindow) -> None:
+    def main_window(self, window: MainWindow | None) -> None:
         self._main_window = window
         self._impl.set_main_window(window)
 
-    def _verify_startup(self):
+    def _verify_startup(self) -> None:
         if not isinstance(self.main_window, MainWindow):
             raise ValueError(
                 "Application does not have a main window. "
                 "Does your startup() method assign a value to self.main_window?"
             )
 
-    def _startup(self):
+    def _startup(self) -> None:
         # This is a wrapper around the user's startup method that performs any
         # post-setup validation.
         self.startup()
@@ -684,7 +672,7 @@ def camera(self) -> Camera:
         return self._camera
 
     @property
-    def commands(self) -> MutableSet[Command]:
+    def commands(self) -> CommandSet:
         """The commands available in the app."""
         return self._commands
 
@@ -716,7 +704,7 @@ def screens(self) -> list[Screen]:
         return [screen.interface for screen in self._impl.get_screens()]
 
     @property
-    def widgets(self) -> Mapping[str, Widget]:
+    def widgets(self) -> WidgetRegistry:
         """The widgets managed by the app, over all windows.
 
         Can be used to look up widgets by ID over the entire app (e.g.,
@@ -729,7 +717,7 @@ def widgets(self) -> Mapping[str, Widget]:
         return self._widgets
 
     @property
-    def windows(self) -> Collection[Window]:
+    def windows(self) -> WindowSet:
         """The windows managed by the app. Windows are automatically added to the app
         when they are created, and removed when they are closed."""
         return self._windows
@@ -783,7 +771,7 @@ def current_window(self) -> Window | None:
         return window.interface
 
     @current_window.setter
-    def current_window(self, window: Window):
+    def current_window(self, window: Window) -> None:
         """Set a window into current active focus."""
         self._impl.set_current_window(window)
 
@@ -829,7 +817,7 @@ def on_exit(self) -> OnExitHandler:
 
     @on_exit.setter
     def on_exit(self, handler: OnExitHandler | None) -> None:
-        def cleanup(app, should_exit):
+        def cleanup(app: App, should_exit: bool) -> None:
             if should_exit or handler is None:
                 app.exit()
 
@@ -851,7 +839,7 @@ def name(self) -> str:
 
     # Support WindowSet __iadd__ and __isub__
     @windows.setter
-    def windows(self, windows):
+    def windows(self, windows: WindowSet) -> None:
         if windows is not self._windows:
             raise AttributeError("can't set attribute 'windows'")
 
@@ -867,15 +855,15 @@ def __init__(
         app_id: str | None = None,
         app_name: str | None = None,
         *,
-        icon: IconContent | None = None,
+        icon: IconContentT | None = None,
         author: str | None = None,
         version: str | None = None,
         home_page: str | None = None,
         description: str | None = None,
         startup: AppStartupMethod | None = None,
-        document_types: dict[str, type[Document]] = None,
+        document_types: dict[str, type[Document]] | None = None,
         on_exit: OnExitHandler | None = None,
-        id=None,  # DEPRECATED
+        id: None = None,  # DEPRECATED
     ):
         """Create a document-based application.
 
@@ -889,7 +877,7 @@ def __init__(
             raise ValueError("A document must manage at least one document type.")
 
         self._document_types = document_types
-        self._documents = []
+        self._documents: list[Document] = []
 
         super().__init__(
             formal_name=formal_name,
@@ -905,10 +893,10 @@ def __init__(
             id=id,
         )
 
-    def _create_impl(self):
-        return self.factory.DocumentApp(interface=self)
+    def _create_impl(self) -> None:
+        self.factory.DocumentApp(interface=self)
 
-    def _verify_startup(self):
+    def _verify_startup(self) -> None:
         # No post-startup validation required for DocumentApps
         pass
 
@@ -934,7 +922,7 @@ def startup(self) -> None:
         Subclasses can override this method to define customized startup behavior.
         """
 
-    def _open(self, path):
+    def _open(self, path: Path) -> None:
         """Internal utility method; open a new document in this app, and shows the document.
 
         :param path: The path to the document to be opened.
diff --git a/core/src/toga/command.py b/core/src/toga/command.py
index 3963550b03..ba6bb5f956 100644
--- a/core/src/toga/command.py
+++ b/core/src/toga/command.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, Protocol
+from collections.abc import Iterator
+from typing import TYPE_CHECKING, Protocol
 
 from toga.handlers import wrapped_handler
 from toga.icons import Icon
@@ -9,7 +10,7 @@
 
 if TYPE_CHECKING:
     from toga.app import App
-    from toga.icons import IconContent
+    from toga.icons import IconContentT
 
 
 class Group:
@@ -40,7 +41,7 @@ def __init__(
 
         # Prime the underlying value of _parent so that the setter has a current value
         # to work with
-        self._parent = None
+        self._parent: Group | None = None
         self.parent = parent
 
     @property
@@ -49,7 +50,7 @@ def parent(self) -> Group | None:
         return self._parent
 
     @parent.setter
-    def parent(self, parent: Group | None):
+    def parent(self, parent: Group | None) -> None:
         if parent is None:
             self._parent = None
         elif parent == self:
@@ -97,17 +98,17 @@ def is_child_of(self, parent: Group | None) -> bool:
     def __hash__(self) -> int:
         return hash(self.key)
 
-    def __lt__(self, other: Any) -> bool:
+    def __lt__(self, other: object) -> bool:
         if not isinstance(other, (Group, Command)):
             return False
         return self.key < other.key
 
-    def __gt__(self, other: Any) -> bool:
+    def __gt__(self, other: object) -> bool:
         if not isinstance(other, (Group, Command)):
             return False
         return other < self
 
-    def __eq__(self, other: Any) -> bool:
+    def __eq__(self, other: object) -> bool:
         if not isinstance(other, (Group, Command)):
             return False
         return self.key == other.key
@@ -121,7 +122,7 @@ def __repr__(self) -> str:
         return f"<Group text={self.text!r} order={self.order}{parent_string}>"
 
     @property
-    def key(self) -> tuple[(int, int, str)]:
+    def key(self) -> tuple[tuple[int, int, str], ...]:
         """A unique tuple describing the path to this group."""
         self_tuple = (self.section, self.order, self.text)
         if self.parent is None:
@@ -130,13 +131,13 @@ def key(self) -> tuple[(int, int, str)]:
 
     # Standard groups - docstrings can only be provided within the `class` statement,
     # but the objects can't be instantiated here.
-    APP = None  #: Application-level commands
-    FILE = None  #: File commands
-    EDIT = None  #: Editing commands
-    VIEW = None  #: Content appearance commands
-    COMMANDS = None  #: Default group for user-provided commands
-    WINDOW = None  #: Window management commands
-    HELP = None  #: Help commands
+    APP: Group  #: Application-level commands
+    FILE: Group  #: File commands
+    EDIT: Group  #: Editing commands
+    VIEW: Group  #: Content appearance commands
+    COMMANDS: Group  #: Default group for user-provided commands
+    WINDOW: Group  #: Window management commands
+    HELP: Group  #: Help commands
 
 
 Group.APP = Group("*", order=0)
@@ -156,7 +157,6 @@ def __call__(self, command: Command, **kwargs) -> bool:
         :param kwargs: Ensures compatibility with additional arguments introduced in
             future versions.
         """
-        ...
 
 
 class Command:
@@ -167,7 +167,7 @@ def __init__(
         *,
         shortcut: str | Key | None = None,
         tooltip: str | None = None,
-        icon: IconContent | None = None,
+        icon: IconContentT | None = None,
         group: Group = Group.COMMANDS,
         section: int = 0,
         order: int = 0,
@@ -184,7 +184,7 @@ def __init__(
         :param text: A label for the command.
         :param shortcut: A key combination that can be used to invoke the command.
         :param tooltip: A short description of what the command will do.
-        :param icon: The :any:`icon content <IconContent>` that can be used to decorate
+        :param icon: The :any:`icon content <IconContentT>` that can be used to decorate
             the command if the platform requires.
         :param group: The group to which this command belongs.
         :param section: The section where the command should appear within its group.
@@ -208,10 +208,11 @@ def __init__(
         self.factory = get_platform_factory()
         self._impl = self.factory.Command(interface=self)
 
+        self._enabled = True
         self.enabled = enabled
 
     @property
-    def key(self) -> tuple[(int, int, str)]:
+    def key(self) -> tuple[tuple[int, int, str], ...]:
         """A unique tuple describing the path to this command.
 
         Each element in the tuple describes the (section, order, text) for the
@@ -225,7 +226,7 @@ def enabled(self) -> bool:
         return self._enabled
 
     @enabled.setter
-    def enabled(self, value: bool):
+    def enabled(self, value: bool) -> None:
         self._enabled = value and getattr(self.action, "_raw", True) is not None
         self._impl.set_enabled(value)
 
@@ -233,12 +234,12 @@ def enabled(self, value: bool):
     def icon(self) -> Icon | None:
         """The Icon for the command.
 
-        Can be specified as any valid :any:`icon content <IconContent>`.
+        Can be specified as any valid :any:`icon content <IconContentT>`.
         """
         return self._icon
 
     @icon.setter
-    def icon(self, icon_or_name: IconContent | None):
+    def icon(self, icon_or_name: IconContentT | None) -> None:
         if isinstance(icon_or_name, Icon) or icon_or_name is None:
             self._icon = icon_or_name
         else:
@@ -250,24 +251,24 @@ def action(self) -> ActionHandler | None:
         return self._action
 
     @action.setter
-    def action(self, action: ActionHandler | None):
+    def action(self, action: ActionHandler | None) -> None:
         """Set the action attached to the command
 
         Needs to be a valid ActionHandler or ``None``
         """
         self._action = wrapped_handler(self, action)
 
-    def __lt__(self, other: Any) -> bool:
+    def __lt__(self, other: object) -> bool:
         if not isinstance(other, (Group, Command)):
             return False
         return self.key < other.key
 
-    def __gt__(self, other: Any) -> bool:
+    def __gt__(self, other: object) -> bool:
         if not isinstance(other, (Group, Command)):
             return False
         return other < self
 
-    def __repr__(self) -> bool:
+    def __repr__(self) -> str:
         return (
             f"<Command text={self.text!r} "
             f"group={self.group} "
@@ -277,7 +278,7 @@ def __repr__(self) -> bool:
 
 
 class Separator:
-    def __init__(self, group: Group = None):
+    def __init__(self, group: Group | None = None):
         """A representation of a separator between sections in a Group.
 
         :param group: The group that contains the separator.
@@ -285,24 +286,27 @@ def __init__(self, group: Group = None):
         self.group = group
 
     def __repr__(self) -> str:
-        return f"<Separator group={self.group.text}>"
+        return f"<Separator group={None if self.group is None else self.group.text}>"
 
-    def __eq__(self, other) -> bool:
+    def __eq__(self, other: object) -> bool:
         if isinstance(other, Separator):
             return self.group == other.group
         return False
 
 
 class CommandSetChangeHandler(Protocol):
-    def __call__(self) -> None:
-        """A handler that will be invoked when a Command or Group is added to the CommandSet."""
-        ...
+    def __call__(self, **kwargs) -> object:
+        """A handler that will be invoked when a Command or Group is added to the
+        CommandSet.
+
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class CommandSet:
     def __init__(
         self,
-        on_change: CommandSetChangeHandler = None,
+        on_change: CommandSetChangeHandler | None = None,
         app: App | None = None,
     ):
         """
@@ -318,35 +322,35 @@ def __init__(
 
         :param on_change: A method that should be invoked when this command set changes.
         :param app: The app this command set is associated with, if it is not the app's
-            own commandset.
+            own :any:`CommandSet`.
         """
         self._app = app
-        self._commands = set()
+        self._commands: set[Command | Group] = set()
         self.on_change = on_change
 
-    def add(self, *commands: Command | Group):
-        if self.app and self.app is not None:
+    def add(self, *commands: Command | Group) -> None:
+        if self.app:
             self.app.commands.add(*commands)
         self._commands.update(commands)
         if self.on_change:
             self.on_change()
 
-    def clear(self):
+    def clear(self) -> None:
         self._commands = set()
         if self.on_change:
             self.on_change()
 
     @property
-    def app(self) -> App:
+    def app(self) -> App | None:
         return self._app
 
     def __len__(self) -> int:
         return len(self._commands)
 
-    def __iter__(self) -> Command | Separator:
+    def __iter__(self) -> Iterator[Command | Separator]:
         cmd_iter = iter(sorted(self._commands))
 
-        def descendant(group, ancestor):
+        def descendant(group: Group, ancestor: Group) -> Group | None:
             # Return the immediate descendant of ancestor used by this group.
             if group.parent == ancestor:
                 return group
diff --git a/core/src/toga/constants/__init__.py b/core/src/toga/constants/__init__.py
index 1f7f3cf47b..8c87e451d0 100644
--- a/core/src/toga/constants/__init__.py
+++ b/core/src/toga/constants/__init__.py
@@ -4,13 +4,15 @@
 
 
 class Direction(Enum):
-    "The direction a given property should act"
+    """The direction a given property should act."""
+
     HORIZONTAL = 0
     VERTICAL = 1
 
 
 class Baseline(Enum):
-    "The meaning of a Y coordinate when drawing text."
+    """The meaning of a Y coordinate when drawing text."""
+
     ALPHABETIC = auto()  #: Alphabetic baseline of the first line
     TOP = auto()  #: Top of text
     MIDDLE = auto()  #: Middle of text
@@ -18,7 +20,8 @@ class Baseline(Enum):
 
 
 class FillRule(Enum):
-    "The rule to use when filling paths."
+    """The rule to use when filling paths."""
+
     EVENODD = 0
     NONZERO = 1
 
@@ -36,7 +39,7 @@ class FlashMode(Enum):
     OFF = 0
     ON = 1
 
-    def __str__(self):
+    def __str__(self) -> str:
         return self.name.title()
 
 
diff --git a/core/src/toga/documents.py b/core/src/toga/documents.py
index a4a02dd7c7..c107896249 100644
--- a/core/src/toga/documents.py
+++ b/core/src/toga/documents.py
@@ -16,7 +16,7 @@ def __init__(
         self,
         path: str | Path,
         document_type: str,
-        app: App = None,
+        app: App,
     ):
         """Create a new Document.
 
@@ -27,7 +27,7 @@ def __init__(
         self._path = Path(path)
         self._document_type = document_type
         self._app = app
-        self._main_window = None
+        self._main_window: Window | None = None
 
         # Create the visual representation of the document
         self.create()
@@ -46,7 +46,7 @@ def can_close(self) -> bool:
         """
         return True
 
-    async def handle_close(self, window, **kwargs):
+    async def handle_close(self, window: Window, **kwargs: object) -> bool:
         """An ``on-close`` handler for the main window of this document that implements
         platform-specific document close behavior.
 
@@ -82,7 +82,7 @@ def filename(self) -> Path:
         return self._path
 
     @property
-    def document_type(self) -> Path:
+    def document_type(self) -> str:
         """A human-readable description of the document type (read-only)."""
         return self._document_type
 
@@ -92,12 +92,12 @@ def app(self) -> App:
         return self._app
 
     @property
-    def main_window(self) -> Window:
+    def main_window(self) -> Window | None:
         """The main window for the document."""
         return self._main_window
 
     @main_window.setter
-    def main_window(self, window):
+    def main_window(self, window: Window) -> None:
         self._main_window = window
 
     def show(self) -> None:
diff --git a/core/src/toga/fonts.py b/core/src/toga/fonts.py
index bbb1042dce..f2a7016d49 100644
--- a/core/src/toga/fonts.py
+++ b/core/src/toga/fonts.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from pathlib import Path
+
 # Use the Travertino font definitions as-is
 from travertino import constants
 from travertino.constants import (
@@ -27,7 +29,7 @@
 FONT_STYLES = {NORMAL, ITALIC, OBLIQUE}
 FONT_VARIANTS = {NORMAL, SMALL_CAPS}
 
-_REGISTERED_FONT_CACHE = {}
+_REGISTERED_FONT_CACHE: dict[tuple[str, str, str, str], str] = {}
 
 
 class Font(BaseFont):
@@ -68,7 +70,14 @@ def __str__(self) -> str:
         return f"{self.family} {size}{weight}{variant}{style}"
 
     @staticmethod
-    def register(family, path, *, weight=NORMAL, style=NORMAL, variant=NORMAL):
+    def register(
+        family: str,
+        path: str | Path,
+        *,
+        weight: str = NORMAL,
+        style: str = NORMAL,
+        variant: str = NORMAL,
+    ) -> None:
         """Registers a file-based font.
 
         **Note:** This is not currently supported on macOS or iOS.
@@ -84,7 +93,12 @@ def register(family, path, *, weight=NORMAL, style=NORMAL, variant=NORMAL):
         _REGISTERED_FONT_CACHE[font_key] = str(toga.App.app.paths.app / path)
 
     @staticmethod
-    def _registered_font_key(family, weight, style, variant):
+    def _registered_font_key(
+        family: str,
+        weight: str,
+        style: str,
+        variant: str,
+    ) -> tuple[str, str, str, str]:
         if weight not in constants.FONT_WEIGHTS:
             weight = NORMAL
         if style not in constants.FONT_STYLES:
diff --git a/core/src/toga/handlers.py b/core/src/toga/handlers.py
index c111473ae6..64a1ea2a74 100644
--- a/core/src/toga/handlers.py
+++ b/core/src/toga/handlers.py
@@ -1,17 +1,51 @@
+from __future__ import annotations
+
 import asyncio
 import inspect
 import sys
 import traceback
 import warnings
 from abc import ABC
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Awaitable,
+    Callable,
+    Generator,
+    NoReturn,
+    Protocol,
+    TypeVar,
+    Union,
+)
+
+if TYPE_CHECKING:
+    if sys.version_info < (3, 10):
+        from typing_extensions import TypeAlias
+    else:
+        from typing import TypeAlias
+
+    GeneratorReturnT = TypeVar("GeneratorReturnT")
+    HandlerGeneratorReturnT: TypeAlias = Generator[
+        Union[float, None], object, GeneratorReturnT
+    ]
+
+    HandlerSyncT: TypeAlias = Callable[..., object]
+    HandlerAsyncT: TypeAlias = Callable[..., Awaitable[object]]
+    HandlerGeneratorT: TypeAlias = Callable[..., HandlerGeneratorReturnT[object]]
+    HandlerT: TypeAlias = Union[HandlerSyncT, HandlerAsyncT, HandlerGeneratorT]
+    WrappedHandlerT: TypeAlias = Callable[..., object]
 
 
 class NativeHandler:
-    def __init__(self, handler):
+    def __init__(self, handler: Callable[..., object]):
         self.native = handler
 
 
-async def long_running_task(interface, generator, cleanup):
+async def long_running_task(
+    interface: object,
+    generator: HandlerGeneratorReturnT[object],
+    cleanup: HandlerSyncT | None,
+) -> None:
     """Run a generator as an asynchronous coroutine."""
     try:
         try:
@@ -33,7 +67,13 @@ async def long_running_task(interface, generator, cleanup):
                 traceback.print_exc()
 
 
-async def handler_with_cleanup(handler, cleanup, interface, *args, **kwargs):
+async def handler_with_cleanup(
+    handler: HandlerAsyncT,
+    cleanup: HandlerSyncT | None,
+    interface: object,
+    *args: object,
+    **kwargs: object,
+) -> None:
     try:
         result = await handler(interface, *args, **kwargs)
     except Exception as e:
@@ -48,7 +88,11 @@ async def handler_with_cleanup(handler, cleanup, interface, *args, **kwargs):
                 traceback.print_exc()
 
 
-def wrapped_handler(interface, handler, cleanup=None):
+def wrapped_handler(
+    interface: object,
+    handler: HandlerT | NativeHandler | None,
+    cleanup: HandlerSyncT | None = None,
+) -> WrappedHandlerT:
     """Wrap a handler provided by the user, so it can be invoked.
 
     If the handler is a NativeHandler, return the handler object contained in the
@@ -70,7 +114,7 @@ def wrapped_handler(interface, handler, cleanup=None):
         if isinstance(handler, NativeHandler):
             return handler.native
 
-        def _handler(*args, **kwargs):
+        def _handler(*args: object, **kwargs: object) -> object:
             if asyncio.iscoroutinefunction(handler):
                 asyncio.ensure_future(
                     handler_with_cleanup(handler, cleanup, interface, *args, **kwargs)
@@ -94,35 +138,47 @@ def _handler(*args, **kwargs):
                         except Exception as e:
                             print("Error in handler cleanup:", e, file=sys.stderr)
                             traceback.print_exc()
+            return None
 
         _handler._raw = handler
 
     else:
         # A dummy no-op handler
-        def _handler(*args, **kwargs):
+        def _handler(*args: object, **kwargs: object) -> object:
             try:
                 if cleanup:
                     cleanup(interface, None)
             except Exception as e:
                 print("Error in handler cleanup:", e, file=sys.stderr)
                 traceback.print_exc()
+            return None
 
         _handler._raw = None
 
     return _handler
 
 
+class OnResultT(Protocol):
+    def __call__(
+        self, result: Any, /, exception: Exception | None = None
+    ) -> object: ...
+
+
 class AsyncResult(ABC):
-    def __init__(self, on_result=None):
+    RESULT_TYPE: str
+
+    def __init__(self, on_result: OnResultT | None = None) -> None:
         loop = asyncio.get_event_loop()
         self.future = loop.create_future()
 
         ######################################################################
         # 2023-12: Backwards compatibility
         ######################################################################
+        self.on_result: OnResultT | None
         if on_result:
             warnings.warn(
-                "Synchronous `on_result` handlers have been deprecated; use `await` on the asynchronous result",
+                "Synchronous `on_result` handlers have been deprecated; "
+                "use `await` on the asynchronous result",
                 DeprecationWarning,
             )
 
@@ -133,26 +189,26 @@ def __init__(self, on_result=None):
         # End backwards compatibility.
         ######################################################################
 
-    def set_result(self, result):
+    def set_result(self, result: object) -> None:
         if not self.future.cancelled():
             self.future.set_result(result)
             if self.on_result:
                 self.on_result(result)
 
-    def set_exception(self, exc):
+    def set_exception(self, exc: Exception) -> None:
         if not self.future.cancelled():
             self.future.set_exception(exc)
             if self.on_result:
                 self.on_result(None, exception=exc)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"<Async {self.RESULT_TYPE} result; future={self.future}>"
 
-    def __await__(self):
+    def __await__(self) -> Generator[Any, None, Any]:
         return self.future.__await__()
 
     # All the comparison dunder methods are disabled
-    def __bool__(self, other):
+    def __bool__(self, other: object) -> NoReturn:
         raise RuntimeError(
             f"Can't check {self.RESULT_TYPE} result directly; use await or an on_result handler"
         )
diff --git a/core/src/toga/hardware/camera.py b/core/src/toga/hardware/camera.py
index 08d4ec430e..d0676727d0 100644
--- a/core/src/toga/hardware/camera.py
+++ b/core/src/toga/hardware/camera.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 from toga.constants import FlashMode
 from toga.handlers import AsyncResult, PermissionResult
@@ -8,6 +8,7 @@
 
 if TYPE_CHECKING:
     from toga.app import App
+    from toga.widgets.base import Widget
 
 
 class PhotoResult(AsyncResult):
@@ -15,7 +16,7 @@ class PhotoResult(AsyncResult):
 
 
 class CameraDevice:
-    def __init__(self, impl):
+    def __init__(self, impl: Any):
         self._impl = impl
 
     @property
@@ -33,7 +34,7 @@ def has_flash(self) -> bool:
         """Does the device have a flash?"""
         return self._impl.has_flash()
 
-    def __eq__(self, other) -> bool:
+    def __eq__(self, other: Widget) -> bool:
         return self.id == other.id
 
     def __repr__(self) -> str:
diff --git a/core/src/toga/hardware/location.py b/core/src/toga/hardware/location.py
index 1a52edd173..56f9801c0e 100644
--- a/core/src/toga/hardware/location.py
+++ b/core/src/toga/hardware/location.py
@@ -17,11 +17,12 @@ class LocationResult(AsyncResult):
 class OnLocationChangeHandler(Protocol):
     def __call__(
         self,
+        *,
         service: Location,
         location: toga.LatLng,
         altitude: float | None,
         **kwargs: Any,
-    ) -> None:
+    ) -> object:
         """A handler that will be invoked when the user's location changes.
 
         :param service: the location service that generated the update.
@@ -30,7 +31,6 @@ def __call__(
             None if the altitude could not be determined.
         :param kwargs: Ensures compatibility with arguments added in future versions.
         """
-        ...
 
 
 class Location:
@@ -137,10 +137,10 @@ def on_change(self) -> OnLocationChangeHandler:
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: OnLocationChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
-    def start_tracking(self):
+    def start_tracking(self) -> None:
         """Start monitoring the user's location for changes.
 
         An :any:`on_change` callback will be generated when the user's location
@@ -156,7 +156,7 @@ def start_tracking(self):
                 "App does not have permission to use location services"
             )
 
-    def stop_tracking(self):
+    def stop_tracking(self) -> None:
         """Stop monitoring the user's location.
 
         :raises PermissionError: If the app has not requested and received permission to
diff --git a/core/src/toga/icons.py b/core/src/toga/icons.py
index 729ae4c09e..8d45474eb7 100644
--- a/core/src/toga/icons.py
+++ b/core/src/toga/icons.py
@@ -2,6 +2,7 @@
 
 import sys
 import warnings
+from collections.abc import Callable, Iterable
 from pathlib import Path
 from typing import TYPE_CHECKING
 
@@ -14,21 +15,18 @@
     else:
         from typing import TypeAlias
 
-    IconContent: TypeAlias = str | Path | toga.Icon
+    IconContentT: TypeAlias = str | Path | toga.Icon
 
 
 class cachedicon:
-    def __init__(self, f):
+    def __init__(self, f: Callable[..., Icon]):
         self.f = f
         self.__doc__ = f.__doc__
 
-    def __get__(self, obj, owner):
+    def __get__(self, obj: object, owner: type[Icon]) -> Icon:
         # If you ask for Icon.CACHED_ICON, obj is None, and owner is the Icon class
         # If you ask for self.CACHED_ICON, obj is self, from which we can get the class.
-        if obj is None:
-            cls = owner
-        else:
-            cls = obj.__class__
+        cls = owner if obj is None else obj.__class__
 
         try:
             # Look for a __CACHED_ICON attribute on the class
@@ -111,6 +109,7 @@ def __init__(
             else:
                 resource_path = toga.App.app.paths.app
 
+            full_path: dict[str, Path] | Path
             if self.factory.Icon.SIZES:
                 full_path = {}
                 for size in self.factory.Icon.SIZES:
@@ -154,7 +153,12 @@ def __init__(
                 )
                 self._impl = self.DEFAULT_ICON._impl
 
-    def _full_path(self, size, extensions, resource_path):
+    def _full_path(
+        self,
+        size: str | None,
+        extensions: Iterable[str],
+        resource_path: Path,
+    ) -> Path:
         platform = toga.platform.current_platform
         if size:
             for extension in extensions:
@@ -178,5 +182,5 @@ def _full_path(self, size, extensions, resource_path):
 
         raise FileNotFoundError(f"Can't find icon {self.path}")
 
-    def __eq__(self, other):
+    def __eq__(self, other: object) -> bool:
         return isinstance(other, Icon) and other._impl.path == self._impl.path
diff --git a/core/src/toga/images.py b/core/src/toga/images.py
index 0d374ce73c..eb4d9fc668 100644
--- a/core/src/toga/images.py
+++ b/core/src/toga/images.py
@@ -1,11 +1,12 @@
 from __future__ import annotations
 
 import importlib
+import os
 import sys
 import warnings
 from functools import lru_cache
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Protocol
+from typing import TYPE_CHECKING, Any, Protocol, TypeVar
 from warnings import warn
 
 import toga
@@ -23,18 +24,18 @@
 
 if TYPE_CHECKING:
     if sys.version_info < (3, 10):
-        from typing_extensions import TypeAlias, TypeVar
+        from typing_extensions import TypeAlias
     else:
-        from typing import TypeAlias, TypeVar
+        from typing import TypeAlias
 
     # Define a type variable for generics where an Image type is required.
     ImageT = TypeVar("ImageT")
 
     # Define the types that can be used as Image content
-    PathLike: TypeAlias = str | Path
-    BytesLike: TypeAlias = bytes | bytearray | memoryview
-    ImageLike: TypeAlias = Any
-    ImageContent: TypeAlias = PathLike | BytesLike | ImageLike
+    PathLikeT: TypeAlias = str | os.PathLike
+    BytesLikeT: TypeAlias = bytes | bytearray | memoryview
+    ImageLikeT: TypeAlias = Any
+    ImageContentT: TypeAlias = PathLikeT | BytesLikeT | ImageLikeT
 
     # Define a type variable representing an image of an externally defined type.
     ExternalImageT = TypeVar("ExternalImageT")
@@ -49,7 +50,7 @@ class ImageConverter(Protocol):
     image_class: type[ExternalImageT]
 
     @staticmethod
-    def convert_from_format(image_in_format: ExternalImageT) -> BytesLike:
+    def convert_from_format(image_in_format: ExternalImageT) -> BytesLikeT:
         """Convert from :any:`image_class` to data in a :ref:`known image format
         <known-image-formats>`.
 
@@ -58,11 +59,10 @@ def convert_from_format(image_in_format: ExternalImageT) -> BytesLike:
         :param image_in_format: An instance of :any:`image_class` (or a subclass).
         :returns: The image data, in a :ref:`known image format <known-image-formats>`.
         """
-        ...
 
     @staticmethod
     def convert_to_format(
-        data: BytesLike,
+        data: BytesLikeT,
         image_class: type[ExternalImageT],
     ) -> ExternalImageT:
         """Convert from data to :any:`image_class` or specified subclass.
@@ -76,7 +76,6 @@ def convert_to_format(
         :param image_class: The class of image to return.
         :returns: The image, as an instance of the image class specified.
         """
-        ...
 
 
 NOT_PROVIDED = object()
@@ -85,15 +84,15 @@ def convert_to_format(
 class Image:
     def __init__(
         self,
-        src: ImageContent = NOT_PROVIDED,
+        src: ImageContentT = NOT_PROVIDED,
         *,
-        path=NOT_PROVIDED,  # DEPRECATED
-        data=NOT_PROVIDED,  # DEPRECATED
+        path: object = NOT_PROVIDED,  # DEPRECATED
+        data: object = NOT_PROVIDED,  # DEPRECATED
     ):
         """Create a new image.
 
         :param src: The source from which to load the image. Can be any valid
-            :any:`image content <ImageContent>` type.
+            :any:`image content <ImageContentT>` type.
         :param path: **DEPRECATED** - Use ``src``.
         :param data: **DEPRECATED** - Use ``src``.
         :raises FileNotFoundError: If a path is provided, but that path does not exist.
@@ -157,7 +156,7 @@ def __init__(
 
     @classmethod
     @lru_cache(maxsize=None)
-    def _converters(cls):
+    def _converters(cls) -> list[ImageConverter]:
         """Return list of registered image plugin converters. Only loaded once."""
         converters = []
 
@@ -172,9 +171,9 @@ def _converters(cls):
         return converters
 
     @property
-    def size(self) -> (int, int):
+    def size(self) -> tuple[int, int]:
         """The size of the image, as a (width, height) tuple."""
-        return (self._impl.get_width(), self._impl.get_height())
+        return self._impl.get_width(), self._impl.get_height()
 
     @property
     def width(self) -> int:
diff --git a/core/src/toga/keys.py b/core/src/toga/keys.py
index bbaa8c1942..4e518cb50c 100644
--- a/core/src/toga/keys.py
+++ b/core/src/toga/keys.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from enum import Enum
 
 
@@ -158,7 +160,7 @@ def is_printable(self) -> bool:
         """Does pressing the key result in a printable character?"""
         return not (self.value.startswith("<") and self.value.endswith(">"))
 
-    def __add__(self, other):
+    def __add__(self, other: Key | str) -> str:
         """Allow two Keys to be concatenated, or a string to be concatenated to a Key.
 
         Produces a single string definition.
@@ -173,6 +175,6 @@ def __add__(self, other):
         except AttributeError:
             return self.value + other
 
-    def __radd__(self, other):
+    def __radd__(self, other: str) -> str:
         """Same as add."""
         return other + self.value
diff --git a/core/src/toga/paths.py b/core/src/toga/paths.py
index a8c38301dd..b73fa03924 100644
--- a/core/src/toga/paths.py
+++ b/core/src/toga/paths.py
@@ -1,4 +1,4 @@
-import importlib
+import importlib.util
 import sys
 from pathlib import Path
 
diff --git a/core/src/toga/platform.py b/core/src/toga/platform.py
index 3c319385d8..d950e29149 100644
--- a/core/src/toga/platform.py
+++ b/core/src/toga/platform.py
@@ -1,11 +1,14 @@
+from __future__ import annotations
+
 import importlib
 import os
 import sys
 from functools import lru_cache
+from types import ModuleType
 
-if sys.version_info >= (3, 10):
+if sys.version_info >= (3, 10):  # pragma: no cover
     from importlib.metadata import entry_points
-else:
+else:  # pragma: no cover
     # Before Python 3.10, entry_points did not support the group argument;
     # so, the backport package must be used on older versions.
     from importlib_metadata import entry_points
@@ -26,7 +29,7 @@
 }
 
 
-def get_current_platform():
+def get_current_platform() -> str | None:
     # Rely on `sys.getandroidapilevel`, which only exists on Android; see
     # https://github.com/beeware/Python-Android-support/issues/8
     if hasattr(sys, "getandroidapilevel"):
@@ -49,7 +52,7 @@ def find_backends():
 
 
 @lru_cache(maxsize=1)
-def get_platform_factory():
+def get_platform_factory() -> ModuleType:
     """Determine the current host platform and import the platform factory.
 
     If the ``TOGA_BACKEND`` environment variable is set, the factory will be loaded
@@ -59,8 +62,7 @@ def get_platform_factory():
 
     :returns: The factory for the host platform.
     """
-    backend_value = os.environ.get("TOGA_BACKEND")
-    if backend_value:
+    if backend_value := os.environ.get("TOGA_BACKEND"):
         try:
             factory = importlib.import_module(f"{backend_value}.factory")
         except ModuleNotFoundError as e:
diff --git a/core/src/toga/plugins/image_formats.py b/core/src/toga/plugins/image_formats.py
index 40822049d4..4787a3267b 100644
--- a/core/src/toga/plugins/image_formats.py
+++ b/core/src/toga/plugins/image_formats.py
@@ -4,7 +4,7 @@
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
-    from toga.images import BytesLike
+    from toga.images import BytesLikeT
 
 # Presumably, other converter plugins will be included with, or only installed
 # alongside, the packages they're for. But since this is provided in Toga, we need to
@@ -29,8 +29,8 @@ def convert_from_format(image_in_format: PIL.Image.Image) -> bytes:
 
     @staticmethod
     def convert_to_format(
-        data: BytesLike,
-        image_class: type(PIL.Image.Image),
+        data: BytesLikeT,
+        image_class: type[PIL.Image.Image],
     ) -> PIL.Image.Image:
         # PIL Images aren't designed to be subclassed, so no implementation is necessary
         # for a supplied format.
diff --git a/core/src/toga/screens.py b/core/src/toga/screens.py
index 3dc24b8f9c..4129bb90db 100644
--- a/core/src/toga/screens.py
+++ b/core/src/toga/screens.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 from toga.images import Image
 from toga.platform import get_platform_factory
@@ -11,7 +11,7 @@
 
 
 class Screen:
-    def __init__(self, _impl):
+    def __init__(self, _impl: Any):
         self._impl = _impl
         self.factory = get_platform_factory()
 
diff --git a/core/src/toga/sources/__init__.py b/core/src/toga/sources/__init__.py
index c0d20e7308..c12ec189f1 100644
--- a/core/src/toga/sources/__init__.py
+++ b/core/src/toga/sources/__init__.py
@@ -3,3 +3,14 @@
 from .list_source import ListSource, Row  # noqa: F401
 from .tree_source import Node, TreeSource  # noqa: F401
 from .value_source import ValueSource  # noqa: F401
+
+__all__ = [
+    "ListSource",
+    "Listener",
+    "Node",
+    "Row",
+    "Source",
+    "TreeSource",
+    "ValueSource",
+    "to_accessor",
+]
diff --git a/core/src/toga/sources/accessors.py b/core/src/toga/sources/accessors.py
index c080ec8b95..e5f2194cd7 100644
--- a/core/src/toga/sources/accessors.py
+++ b/core/src/toga/sources/accessors.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import re
+from collections.abc import Iterable, Mapping
 
 NON_ACCESSOR_CHARS = re.compile(r"[^\w ]")
 WHITESPACE = re.compile(r"\s+")
@@ -45,8 +46,8 @@ def to_accessor(heading: str) -> str:
 
 
 def build_accessors(
-    headings: list[str],
-    accessors: list[str | None] | dict[str, str] | None,
+    headings: Iterable[str],
+    accessors: Iterable[str | None] | Mapping[str, str] | None,
 ) -> list[str]:
     """Convert a list of headings (with accessor overrides) to a finalised list of
     accessors.
@@ -62,13 +63,14 @@ def build_accessors(
 
     :returns: The final list of accessors.
     """
-    if accessors:
-        if isinstance(accessors, dict):
+    if accessors is not None:
+        if isinstance(accessors, Mapping):
             result = [
                 accessors[h] if h in accessors else to_accessor(h) for h in headings
             ]
         else:
-            if len(headings) != len(accessors):
+            # TODO: use zip(..., strict=True) instead once Python 3.9 support is dropped
+            if len(headings := list(headings)) != len(accessors := list(accessors)):
                 raise ValueError("Number of accessors must match number of headings")
 
             result = [
diff --git a/core/src/toga/sources/base.py b/core/src/toga/sources/base.py
index 06e9d539c0..8ea13d2d5b 100644
--- a/core/src/toga/sources/base.py
+++ b/core/src/toga/sources/base.py
@@ -8,35 +8,35 @@ class Listener(Protocol):
     data source.
     """
 
-    def change(self, item):
+    def change(self, item: object) -> object:
         """A change has occurred in an item.
 
         :param item: The data object that has changed.
         """
 
-    def insert(self, index: int, item):
+    def insert(self, index: int, item: object) -> object:
         """An item has been added to the data source.
 
         :param index: The 0-index position in the data.
         :param item: The data object that was added.
         """
 
-    def remove(self, index: int, item):
+    def remove(self, index: int, item: object) -> object:
         """An item has been removed from the data source.
 
         :param index: The 0-index position in the data.
         :param item: The data object that was added.
         """
 
-    def clear(self):
+    def clear(self) -> object:
         """All items have been removed from the data source."""
 
 
 class Source:
     """A base class for data sources, providing an implementation of data notifications."""
 
-    def __init__(self):
-        self._listeners = []
+    def __init__(self) -> None:
+        self._listeners: list[Listener] = []
 
     @property
     def listeners(self) -> list[Listener]:
@@ -46,7 +46,7 @@ def listeners(self) -> list[Listener]:
         """
         return self._listeners
 
-    def add_listener(self, listener: Listener):
+    def add_listener(self, listener: Listener) -> None:
         """Add a new listener to this data source.
 
         If the listener is already registered on this data source, the
@@ -57,14 +57,14 @@ def add_listener(self, listener: Listener):
         if listener not in self._listeners:
             self._listeners.append(listener)
 
-    def remove_listener(self, listener: Listener):
+    def remove_listener(self, listener: Listener) -> None:
         """Remove a listener from this data source.
 
         :param listener: The listener to remove.
         """
         self._listeners.remove(listener)
 
-    def notify(self, notification: str, **kwargs):
+    def notify(self, notification: str, **kwargs: object) -> None:
         """Notify all listeners an event has occurred.
 
         :param notification: The notification to emit.
diff --git a/core/src/toga/sources/list_source.py b/core/src/toga/sources/list_source.py
index b72e696fb9..666bc71925 100644
--- a/core/src/toga/sources/list_source.py
+++ b/core/src/toga/sources/list_source.py
@@ -1,12 +1,20 @@
 from __future__ import annotations
 
-from collections.abc import Iterable
-from typing import Any
+from collections.abc import Iterable, Mapping, Sequence
+from typing import Generic, TypeVar
 
 from .base import Source
 
+T = TypeVar("T")
 
-def _find_item(candidates: list, data: Any, accessors: list[str], start, error: str):
+
+def _find_item(
+    candidates: Sequence[T],
+    data: object,
+    accessors: Sequence[str],
+    start: T | None,
+    error: str,
+) -> T:
     """Find-by-value implementation helper; find an item matching ``data`` in
     ``candidates``, starting with item ``start``."""
     if start is not None:
@@ -16,7 +24,7 @@ def _find_item(candidates: list, data: Any, accessors: list[str], start, error:
 
     for item in candidates[start_index:]:
         try:
-            if isinstance(data, dict):
+            if isinstance(data, Mapping):
                 found = all(
                     getattr(item, attr) == value for attr, value in data.items()
                 )
@@ -36,8 +44,8 @@ def _find_item(candidates: list, data: Any, accessors: list[str], start, error:
     raise ValueError(error)
 
 
-class Row:
-    def __init__(self, **data):
+class Row(Generic[T]):
+    def __init__(self, **data: T):
         """Create a new Row object.
 
         The keyword arguments specified in the constructor will be converted into
@@ -51,7 +59,7 @@ def __init__(self, **data):
         for name, value in data.items():
             setattr(self, name, value)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         descriptor = " ".join(
             f"{attr}={getattr(self, attr)!r}"
             for attr in sorted(self.__dict__)
@@ -63,7 +71,7 @@ def __repr__(self):
     # Utility wrappers
     ######################################################################
 
-    def __setattr__(self, attr: str, value):
+    def __setattr__(self, attr: str, value: T) -> None:
         """Set an attribute on the Row object, notifying the source of the change.
 
         :param attr: The attribute to change.
@@ -74,11 +82,13 @@ def __setattr__(self, attr: str, value):
             if self._source is not None:
                 self._source.notify("change", item=self)
 
-    def __delattr__(self, attr: str):
+    def __getattr__(self, attr: str) -> T:
+        return super().__getattr__(attr)
+
+    def __delattr__(self, attr: str) -> None:
         """Remove an attribute from the Row object, notifying the source of the change.
 
         :param attr: The attribute to change.
-        :param value: The new attribute value.
         """
         super().__delattr__(attr)
         if not attr.startswith("_"):
@@ -87,7 +97,9 @@ def __delattr__(self, attr: str):
 
 
 class ListSource(Source):
-    def __init__(self, accessors: list[str], data: Iterable | None = None):
+    _data: list[Row]
+
+    def __init__(self, accessors: Iterable[str], data: Iterable | None = None):
         """A data source to store an ordered list of multiple data values.
 
         :param accessors: A list of attribute names for accessing the value
@@ -105,7 +117,7 @@ def __init__(self, accessors: list[str], data: Iterable | None = None):
             raise ValueError("ListSource must be provided a list of accessors")
 
         # Convert the data into row objects
-        if data:
+        if data is not None:
             self._data = [self._create_row(value) for value in data]
         else:
             self._data = []
@@ -122,7 +134,7 @@ def __getitem__(self, index: int) -> Row:
         """Returns the item at position ``index`` of the list."""
         return self._data[index]
 
-    def __delitem__(self, index: int):
+    def __delitem__(self, index: int) -> None:
         """Deletes the item at position ``index`` of the list."""
         row = self._data[index]
         del self._data[index]
@@ -133,8 +145,8 @@ def __delitem__(self, index: int):
     ######################################################################
 
     # This behavior is documented in list_source.rst.
-    def _create_row(self, data: Any) -> Row:
-        if isinstance(data, dict):
+    def _create_row(self, data: object) -> Row:
+        if isinstance(data, Mapping):
             row = Row(**data)
         elif hasattr(data, "__iter__") and not isinstance(data, str):
             row = Row(**dict(zip(self._accessors, data)))
@@ -147,7 +159,7 @@ def _create_row(self, data: Any) -> Row:
     # Utility methods to make ListSources more list-like
     ######################################################################
 
-    def __setitem__(self, index: int, value: Any):
+    def __setitem__(self, index: int, value: object) -> None:
         """Set the value of a specific item in the data source.
 
         :param index: The item to change
@@ -158,12 +170,12 @@ def __setitem__(self, index: int, value: Any):
         self._data[index] = row
         self.notify("insert", index=index, item=row)
 
-    def clear(self):
+    def clear(self) -> None:
         """Clear all data from the data source."""
         self._data = []
         self.notify("clear")
 
-    def insert(self, index: int, data: Any):
+    def insert(self, index: int, data: object) -> Row:
         """Insert a row into the data source at a specific index.
 
         :param index: The index at which to insert the item.
@@ -176,7 +188,7 @@ def insert(self, index: int, data: Any):
         self.notify("insert", index=index, item=row)
         return row
 
-    def append(self, data):
+    def append(self, data: object) -> Row:
         """Insert a row at the end of the data source.
 
         :param data: The data to append to the ListSource. This data will be converted
@@ -185,7 +197,7 @@ def append(self, data):
         """
         return self.insert(len(self), data)
 
-    def remove(self, row: Row):
+    def remove(self, row: Row) -> None:
         """Remove a row from the data source.
 
         :param row: The row to remove from the data source.
@@ -206,7 +218,7 @@ def index(self, row: Row) -> int:
         """
         return self._data.index(row)
 
-    def find(self, data: Any, start: None | None = None):
+    def find(self, data: object, start: Row | None = None) -> Row:
         """Find the first item in the data that matches all the provided
         attributes.
 
diff --git a/core/src/toga/sources/tree_source.py b/core/src/toga/sources/tree_source.py
index 8b4a5268f5..967a97267e 100644
--- a/core/src/toga/sources/tree_source.py
+++ b/core/src/toga/sources/tree_source.py
@@ -1,13 +1,18 @@
 from __future__ import annotations
 
-from typing import Any
+from collections.abc import Iterator
+from typing import Iterable, Mapping, TypeVar
 
 from .base import Source
 from .list_source import Row, _find_item
 
+T = TypeVar("T")
 
-class Node(Row):
-    def __init__(self, **data):
+
+class Node(Row[T]):
+    _source: TreeSource
+
+    def __init__(self, **data: T):
         """Create a new Node object.
 
         The keyword arguments specified in the constructor will be converted into
@@ -21,10 +26,10 @@ def __init__(self, **data):
         notified.
         """
         super().__init__(**data)
-        self._children: list[Node] | None = None
-        self._parent: Node | None = None
+        self._children: list[Node[T]] | None = None
+        self._parent: Node[T] | None = None
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         descriptor = " ".join(
             f"{attr}={getattr(self, attr)!r}"
             for attr in sorted(self.__dict__)
@@ -41,13 +46,13 @@ def __repr__(self):
     # Methods required by the TreeSource interface
     ######################################################################
 
-    def __getitem__(self, index: int) -> Node:
+    def __getitem__(self, index: int) -> Node[T]:
         if self._children is None:
             raise ValueError(f"{self} is a leaf node")
 
         return self._children[index]
 
-    def __delitem__(self, index: int):
+    def __delitem__(self, index: int) -> None:
         if self._children is None:
             raise ValueError(f"{self} is a leaf node")
 
@@ -79,10 +84,10 @@ def can_have_children(self) -> bool:
     # Utility methods to make TreeSource more list-like
     ######################################################################
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator[Node[T]]:
         return iter(self._children or [])
 
-    def __setitem__(self, index: int, data: Any):
+    def __setitem__(self, index: int, data: object) -> None:
         """Set the value of a specific child in the Node.
 
         :param index: The index of the child to change
@@ -100,7 +105,7 @@ def __setitem__(self, index: int, data: Any):
         self._children[index] = node
         self._source.notify("change", item=node)
 
-    def insert(self, index: int, data: Any, children: Any = None):
+    def insert(self, index: int, data: object, children: object = None) -> Node[T]:
         """Insert a node as a child of this node a specific index.
 
         :param index: The index at which to insert the new child.
@@ -122,7 +127,7 @@ def insert(self, index: int, data: Any, children: Any = None):
         self._source.notify("insert", parent=self, index=index, item=node)
         return node
 
-    def append(self, data: Any, children: Any = None):
+    def append(self, data: object, children: object = None) -> Node[T]:
         """Append a node to the end of the list of children of this node.
 
         :param data: The data to append as a child of this node. This data will be
@@ -132,7 +137,7 @@ def append(self, data: Any, children: Any = None):
         """
         return self.insert(len(self), data=data, children=children)
 
-    def remove(self, child: Node):
+    def remove(self, child: Node[T]) -> None:
         """Remove a child node from this node.
 
         :param child: The child node to remove from this node.
@@ -140,7 +145,7 @@ def remove(self, child: Node):
         # Index will raise ValueError if the node is a leaf
         del self[self.index(child)]
 
-    def index(self, child: Node):
+    def index(self, child: Node[T]) -> int:
         """The index of a specific node in children of this node.
 
         This search uses Node instances, and searches for an *instance* match.
@@ -157,7 +162,7 @@ def index(self, child: Node):
 
         return self._children.index(child)
 
-    def find(self, data: Any, start: Node = None):
+    def find(self, data: object, start: Node[T] | None = None) -> Node[T]:
         """Find the first item in the child nodes of this node that matches all the
         provided attributes.
 
@@ -189,7 +194,9 @@ def find(self, data: Any, start: Node = None):
 
 
 class TreeSource(Source):
-    def __init__(self, accessors: list[str], data: dict | list[tuple] | None = None):
+    _roots: list[Node]
+
+    def __init__(self, accessors: Iterable[str], data: object | None = None):
         super().__init__()
         if isinstance(accessors, str) or not hasattr(accessors, "__iter__"):
             raise ValueError("accessors should be a list of attribute names")
@@ -199,7 +206,7 @@ def __init__(self, accessors: list[str], data: dict | list[tuple] | None = None)
         if len(self._accessors) == 0:
             raise ValueError("TreeSource must be provided a list of accessors")
 
-        if data:
+        if data is not None:
             self._roots = self._create_nodes(parent=None, value=data)
         else:
             self._roots = []
@@ -214,7 +221,7 @@ def __len__(self) -> int:
     def __getitem__(self, index: int) -> Node:
         return self._roots[index]
 
-    def __delitem__(self, index: int):
+    def __delitem__(self, index: int) -> None:
         node = self._roots[index]
         del self._roots[index]
         node._source = None
@@ -227,10 +234,10 @@ def __delitem__(self, index: int):
     def _create_node(
         self,
         parent: Node | None,
-        data: Any,
-        children: list | dict | None = None,
-    ):
-        if isinstance(data, dict):
+        data: object,
+        children: object | None = None,
+    ) -> Node:
+        if isinstance(data, Mapping):
             node = Node(**data)
         elif hasattr(data, "__iter__") and not isinstance(data, str):
             node = Node(**dict(zip(self._accessors, data)))
@@ -245,8 +252,8 @@ def _create_node(
 
         return node
 
-    def _create_nodes(self, parent: Node | None, value: Any):
-        if isinstance(value, dict):
+    def _create_nodes(self, parent: Node | None, value: object) -> list[Node]:
+        if isinstance(value, Mapping):
             return [
                 self._create_node(parent=parent, data=data, children=children)
                 for data, children in value.items()
@@ -263,7 +270,7 @@ def _create_nodes(self, parent: Node | None, value: Any):
     # Utility methods to make TreeSources more list-like
     ######################################################################
 
-    def __setitem__(self, index: int, data: Any):
+    def __setitem__(self, index: int, data: object) -> None:
         """Set the value of a specific root item in the data source.
 
         :param index: The root item to change
@@ -278,17 +285,12 @@ def __setitem__(self, index: int, data: Any):
         self._roots[index] = root
         self.notify("change", item=root)
 
-    def clear(self):
+    def clear(self) -> None:
         """Clear all data from the data source."""
         self._roots = []
         self.notify("clear")
 
-    def insert(
-        self,
-        index: int,
-        data: Any,
-        children: Any = None,
-    ):
+    def insert(self, index: int, data: object, children: object = None) -> Node:
         """Insert a root node into the data source at a specific index.
 
         If the node is a leaf node, it will be converted into a non-leaf node.
@@ -309,9 +311,10 @@ def insert(
         self._roots.insert(index, node)
         node._parent = None
         self.notify("insert", parent=None, index=index, item=node)
+
         return node
 
-    def append(self, data: Any, children: Any = None):
+    def append(self, data: object, children: object | None = None) -> Node:
         """Append a root node at the end of the list of children of this source.
 
         If the node is a leaf node, it will be converted into a non-leaf node.
@@ -324,7 +327,7 @@ def append(self, data: Any, children: Any = None):
         """
         return self.insert(len(self), data=data, children=children)
 
-    def remove(self, node: Node):
+    def remove(self, node: Node) -> None:
         """Remove a node from the data source.
 
         This will also remove the node if it is a descendant of a root node.
@@ -353,7 +356,7 @@ def index(self, node: Node) -> int:
         """
         return self._roots.index(node)
 
-    def find(self, data: Any, start: Node = None):
+    def find(self, data: object, start: Node | None = None) -> Node:
         """Find the first item in the child nodes of the given node that matches all the
         provided attributes.
 
diff --git a/core/src/toga/sources/value_source.py b/core/src/toga/sources/value_source.py
index c77fcf5d3a..1d8f10178b 100644
--- a/core/src/toga/sources/value_source.py
+++ b/core/src/toga/sources/value_source.py
@@ -4,15 +4,15 @@
 
 
 class ValueSource(Source):
-    def __init__(self, value=None, accessor="value"):
+    def __init__(self, value: object = None, accessor: str = "value"):
         super().__init__()
         self.accessor = accessor
         setattr(self, accessor, value)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return str(getattr(self, self.accessor, None))
 
-    def __setattr__(self, attr, value):
+    def __setattr__(self, attr: str, value: object) -> None:
         super().__setattr__(attr, value)
         if attr == getattr(self, "accessor", None):
             self.notify("change", item=value)
diff --git a/core/src/toga/style/__init__.py b/core/src/toga/style/__init__.py
index 4605b72b2f..33014fdd05 100644
--- a/core/src/toga/style/__init__.py
+++ b/core/src/toga/style/__init__.py
@@ -1,2 +1,7 @@
 from toga.style.applicator import TogaApplicator  # noqa: F401
 from toga.style.pack import Pack  # noqa: F401
+
+__all__ = [
+    "Pack",
+    "TogaApplicator",
+]
diff --git a/core/src/toga/style/applicator.py b/core/src/toga/style/applicator.py
index 4f9a7552db..2b0c17ee75 100644
--- a/core/src/toga/style/applicator.py
+++ b/core/src/toga/style/applicator.py
@@ -1,14 +1,22 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from toga.widgets.base import Widget
+
+
 class TogaApplicator:
     """Apply styles to a Toga widget."""
 
-    def __init__(self, widget):
+    def __init__(self, widget: Widget):
         self.widget = widget
 
-    def refresh(self):
+    def refresh(self) -> None:
         # print("RE-EVALUATE LAYOUT", self.widget)
         self.widget.refresh()
 
-    def set_bounds(self):
+    def set_bounds(self) -> None:
         # print("  APPLY LAYOUT", self.widget, self.widget.layout)
         self.widget._impl.set_bounds(
             self.widget.layout.absolute_content_left,
@@ -19,10 +27,10 @@ def set_bounds(self):
         for child in self.widget.children:
             child.applicator.set_bounds()
 
-    def set_text_alignment(self, alignment):
+    def set_text_alignment(self, alignment: str) -> None:
         self.widget._impl.set_alignment(alignment)
 
-    def set_hidden(self, hidden):
+    def set_hidden(self, hidden: bool) -> None:
         self.widget._impl.set_hidden(hidden)
         for child in self.widget.children:
             # If the parent is hidden, then so are all children. However, if the
@@ -38,14 +46,14 @@ def set_hidden(self, hidden):
             # False         False              False
             child.applicator.set_hidden(hidden or child.style._hidden)
 
-    def set_font(self, font):
+    def set_font(self, font: object) -> None:
         # Changing the font of a widget can make the widget change size,
         # which in turn means we need to do a re-layout
         self.widget._impl.set_font(font)
         self.widget.refresh()
 
-    def set_color(self, color):
+    def set_color(self, color: object) -> None:
         self.widget._impl.set_color(color)
 
-    def set_background_color(self, color):
+    def set_background_color(self, color: object) -> None:
         self.widget._impl.set_background_color(color)
diff --git a/core/src/toga/style/pack.py b/core/src/toga/style/pack.py
index 455eb2e70f..8743293968 100644
--- a/core/src/toga/style/pack.py
+++ b/core/src/toga/style/pack.py
@@ -1,3 +1,7 @@
+from __future__ import annotations
+
+from typing import Any
+
 from travertino.constants import (  # noqa: F401
     BOLD,
     BOTTOM,
@@ -27,6 +31,7 @@
 )
 from travertino.declaration import BaseStyle, Choices
 from travertino.layout import BaseBox
+from travertino.node import Node
 from travertino.size import BaseIntrinsicSize
 
 from toga.fonts import (
@@ -80,15 +85,15 @@ class IntrinsicSize(BaseIntrinsicSize):
 
     _depth = -1
 
-    def _debug(self, *args):  # pragma: no cover
+    def _debug(self, *args: str) -> None:  # pragma: no cover
         print("    " * self.__class__._depth, *args)
 
     @property
-    def _hidden(self):
-        """Does this style declaration define an object that should be hidden"""
+    def _hidden(self) -> bool:
+        """Does this style declaration define an object that should be hidden."""
         return self.visibility == HIDDEN
 
-    def apply(self, prop, value):
+    def apply(self, prop: str, value: object) -> None:
         if self._applicator:
             if prop == "text_align":
                 if value is None:
@@ -127,7 +132,7 @@ def apply(self, prop, value):
                 # so perform a refresh.
                 self._applicator.refresh()
 
-    def layout(self, node, viewport):
+    def layout(self, node: Node, viewport: Any) -> None:
         # self._debug("=" * 80)
         # self._debug(f"Layout root {node}, available {viewport.width}x{viewport.height}")
         self.__class__._depth = -1
@@ -147,12 +152,12 @@ def layout(self, node, viewport):
 
     def _layout_node(
         self,
-        node,
-        alloc_width,
-        alloc_height,
-        use_all_width,
-        use_all_height,
-    ):
+        node: Node,
+        alloc_width: int,
+        alloc_height: int,
+        use_all_width: bool,
+        use_all_height: bool,
+    ) -> None:
         self.__class__._depth += 1
         # self._debug(
         #     f"COMPUTE LAYOUT for {node} available "
@@ -259,12 +264,12 @@ def _layout_node(
 
     def _layout_row_children(
         self,
-        node,
-        available_width,
-        available_height,
-        use_all_width,
-        use_all_height,
-    ):
+        node: Node,
+        available_width: int,
+        available_height: int,
+        use_all_width: bool,
+        use_all_height: bool,
+    ) -> tuple[int, int, int, int]:
         # self._debug(f"LAYOUT ROW CHILDREN {available_width=} {available_height=}")
         # Pass 1: Lay out all children with a hard-specified width, or an
         # intrinsic non-flexible width. While iterating, collect the flex
@@ -543,12 +548,12 @@ def _layout_row_children(
 
     def _layout_column_children(
         self,
-        node,
-        available_width,
-        available_height,
-        use_all_width,
-        use_all_height,
-    ):
+        node: Node,
+        available_width: int,
+        available_height: int,
+        use_all_width: bool,
+        use_all_height: bool,
+    ) -> tuple[int, int, int, int]:
         # self._debug(f"LAYOUT COLUMN CHILDREN {available_width=} {available_height=}")
         # Pass 1: Lay out all children with a hard-specified height, or an
         # intrinsic non-flexible height. While iterating, collect the flex
@@ -819,7 +824,7 @@ def _layout_column_children(
 
         return min_width, width, min_height, height
 
-    def __css__(self):
+    def __css__(self) -> str:
         css = []
         # display
         if self.display == NONE:
diff --git a/core/src/toga/types.py b/core/src/toga/types.py
index eeacd34de8..b50553cb1f 100644
--- a/core/src/toga/types.py
+++ b/core/src/toga/types.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import sys
-from typing import TYPE_CHECKING, NamedTuple, Tuple
+from typing import TYPE_CHECKING, NamedTuple
 
 import toga
 
@@ -11,8 +11,8 @@
     else:
         from typing import TypeAlias
 
-    PositionT: TypeAlias = toga.Position | Tuple[int, int]
-    SizeT: TypeAlias = toga.Size | Tuple[int, int]
+    PositionT: TypeAlias = toga.Position | tuple[int, int]
+    SizeT: TypeAlias = toga.Size | tuple[int, int]
 
 
 class LatLng(NamedTuple):
@@ -24,7 +24,7 @@ class LatLng(NamedTuple):
     #: Longitude
     lng: float
 
-    def __str__(self):
+    def __str__(self) -> str:
         return f"({self.lat:6f}, {self.lng:6f})"
 
 
diff --git a/core/src/toga/validators.py b/core/src/toga/validators.py
index a68cb6e21e..83ae650919 100644
--- a/core/src/toga/validators.py
+++ b/core/src/toga/validators.py
@@ -30,7 +30,6 @@ def is_valid(self, input_string: str) -> bool:
         :param input_string: The string to validate.
         :returns: ``True`` if the input is valid.
         """
-        ...
 
 
 class CountValidator:
@@ -83,7 +82,6 @@ def count(self, input_string: str) -> int:
         :param input_string: The string to inspect for content of interest.
         :returns: The number of instances of content that the validator is looking for.
         """
-        ...
 
 
 class LengthBetween(BooleanValidator):
@@ -105,8 +103,8 @@ def __init__(
             ``True``
         """
         if error_message is None:
-            error_message = "Input should be between {} and {} characters".format(
-                min_length, max_length
+            error_message = (
+                f"Input should be between {min_length} and {max_length} characters"
             )
         super().__init__(error_message=error_message, allow_empty=allow_empty)
 
@@ -146,9 +144,7 @@ def __init__(
             ``True``
         """
         if error_message is None:
-            error_message = "Input is too short (length should be at least {})".format(
-                length
-            )
+            error_message = f"Input is too short (length should be at least {length})"
         super().__init__(
             min_length=length,
             max_length=None,
@@ -170,9 +166,7 @@ def __init__(
             string is too long.
         """
         if error_message is None:
-            error_message = "Input is too long (length should be at most {})".format(
-                length
-            )
+            error_message = f"Input is too long (length should be at most {length})"
         super().__init__(
             min_length=None,
             max_length=length,
@@ -294,7 +288,7 @@ def __init__(
 class MatchRegex(BooleanValidator):
     def __init__(
         self,
-        regex_string,
+        regex_string: str,
         error_message: str | None = None,
         allow_empty: bool = True,
     ):
@@ -430,9 +424,7 @@ def __init__(
         else:
             expected_existence_message = "Input should contain at least one digit"
             expected_non_existence_message = "Input should not contain digits"
-            expected_count_message = "Input should contain exactly {} digits".format(
-                count
-            )
+            expected_count_message = f"Input should contain exactly {count} digits"
 
         super().__init__(
             count=count,
diff --git a/core/src/toga/widgets/activityindicator.py b/core/src/toga/widgets/activityindicator.py
index 7681ce4fc6..102f44d129 100644
--- a/core/src/toga/widgets/activityindicator.py
+++ b/core/src/toga/widgets/activityindicator.py
@@ -2,14 +2,14 @@
 
 from typing import Literal
 
-from .base import Widget
+from .base import StyleT, Widget
 
 
 class ActivityIndicator(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         running: bool = False,
     ):
         """Create a new ActivityIndicator widget.
@@ -41,7 +41,7 @@ def enabled(self, value: bool) -> None:
         pass
 
     def focus(self) -> None:
-        "No-op; ActivityIndicator cannot accept input focus"
+        """No-op; ActivityIndicator cannot accept input focus."""
         pass
 
     @property
diff --git a/core/src/toga/widgets/base.py b/core/src/toga/widgets/base.py
index bb12d78d96..8a545916d4 100644
--- a/core/src/toga/widgets/base.py
+++ b/core/src/toga/widgets/base.py
@@ -1,8 +1,9 @@
 from __future__ import annotations
 
 from builtins import id as identifier
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any, TypeVar
 
+from travertino.declaration import BaseStyle
 from travertino.node import Node
 
 from toga.platform import get_platform_factory
@@ -12,6 +13,8 @@
     from toga.app import App
     from toga.window import Window
 
+StyleT = TypeVar("StyleT", bound=BaseStyle)
+
 
 class Widget(Node):
     _MIN_WIDTH = 100
@@ -20,7 +23,7 @@ class Widget(Node):
     def __init__(
         self,
         id: str | None = None,
-        style=None,
+        style: StyleT | None = None,
     ):
         """Create a base Toga widget.
 
@@ -36,16 +39,16 @@ def __init__(
         )
 
         self._id = str(id if id else identifier(self))
-        self._window = None
-        self._app = None
-        self._impl = None
+        self._window: Window | None = None
+        self._app: App | None = None
+        self._impl: Any = None
 
         self.factory = get_platform_factory()
 
     def __repr__(self) -> str:
         return f"<{self.__class__.__name__}:0x{identifier(self):x}>"
 
-    def __lt__(self, other) -> bool:
+    def __lt__(self, other: Widget) -> bool:
         return self.id < other.id
 
     @property
@@ -62,8 +65,7 @@ def tab_index(self) -> int | None:
 
         .. note::
 
-            This is a beta feature. The ``tab_index`` API may change in
-            the future.
+            This is a beta feature. The ``tab_index`` API may change in the future.
         """
         return self._impl.get_tab_index()
 
@@ -71,7 +73,7 @@ def tab_index(self) -> int | None:
     def tab_index(self, tab_index: int) -> None:
         self._impl.set_tab_index(tab_index)
 
-    def _assert_can_have_children(self):
+    def _assert_can_have_children(self) -> None:
         if not self.can_have_children:
             raise ValueError(f"{type(self).__name__} cannot have children")
 
diff --git a/core/src/toga/widgets/box.py b/core/src/toga/widgets/box.py
index cc6a61b9ad..5f84160fb1 100644
--- a/core/src/toga/widgets/box.py
+++ b/core/src/toga/widgets/box.py
@@ -1,6 +1,8 @@
 from __future__ import annotations
 
-from .base import Widget
+from collections.abc import Iterable
+
+from .base import StyleT, Widget
 
 
 class Box(Widget):
@@ -10,8 +12,8 @@ class Box(Widget):
     def __init__(
         self,
         id: str | None = None,
-        style=None,
-        children: list[Widget] | None = None,
+        style: StyleT | None = None,
+        children: Iterable[Widget] | None = None,
     ):
         """Create a new Box container widget.
 
@@ -26,8 +28,8 @@ def __init__(
         self._impl = self.factory.Box(interface=self)
 
         # Children need to be added *after* the impl has been created.
-        self._children = []
-        if children:
+        self._children: list[Widget] = []
+        if children is not None:
             self.add(*children)
 
     @property
diff --git a/core/src/toga/widgets/button.py b/core/src/toga/widgets/button.py
index d065425f42..c6ab49b7b1 100644
--- a/core/src/toga/widgets/button.py
+++ b/core/src/toga/widgets/button.py
@@ -5,37 +5,36 @@
 import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
 
 if TYPE_CHECKING:
-    from toga.icons import IconContent
+    from toga.icons import IconContentT
 
 
 class OnPressHandler(Protocol):
-    def __call__(self, widget: Button, **kwargs: Any) -> None:
+    def __call__(self, widget: Button, /, **kwargs: Any) -> object:
         """A handler that will be invoked when a button is pressed.
 
         :param widget: The button that was pressed.
         :param kwargs: Ensures compatibility with arguments added in future versions.
         """
-        ...
 
 
 class Button(Widget):
     def __init__(
         self,
         text: str | None = None,
-        icon: IconContent | None = None,
+        icon: IconContentT | None = None,
         id: str | None = None,
-        style=None,
-        on_press: OnPressHandler | None = None,
+        style: StyleT | None = None,
+        on_press: toga.widgets.button.OnPressHandler | None = None,
         enabled: bool = True,
     ):
         """Create a new button widget.
 
         :param text: The text to display on the button.
         :param icon: The icon to display on the button. Can be specified as any valid
-            :any:`icon content <IconContent>`.
+            :any:`icon content <IconContentT>`.
         :param id: The ID for the widget.
         :param style: A style object. If no style is provided, a default style will be
             applied to the widget.
@@ -85,6 +84,7 @@ def text(self) -> str:
 
     @text.setter
     def text(self, value: str | None) -> None:
+        # \u200B: zero-width space
         if value is None or value == "\u200B":
             value = ""
         else:
@@ -100,7 +100,7 @@ def text(self, value: str | None) -> None:
     def icon(self) -> toga.Icon | None:
         """The icon displayed on the button.
 
-        Can be specified as any valid :any:`icon content <IconContent>`.
+        Can be specified as any valid :any:`icon content <IconContentT>`.
 
         If the button is currently displaying text, and an icon is assigned, the text
         will be replaced by the new icon.
@@ -113,7 +113,7 @@ def icon(self) -> toga.Icon | None:
         return self._impl.get_icon()
 
     @icon.setter
-    def icon(self, value: IconContent | None) -> None:
+    def icon(self, value: IconContentT | None) -> None:
         if isinstance(value, toga.Icon):
             icon = value
             text = ""
@@ -138,5 +138,5 @@ def on_press(self) -> OnPressHandler:
         return self._on_press
 
     @on_press.setter
-    def on_press(self, handler):
+    def on_press(self, handler: toga.widgets.button.OnPressHandler) -> None:
         self._on_press = wrapped_handler(self, handler)
diff --git a/core/src/toga/widgets/canvas.py b/core/src/toga/widgets/canvas.py
index bebf336f90..8adf19c1aa 100644
--- a/core/src/toga/widgets/canvas.py
+++ b/core/src/toga/widgets/canvas.py
@@ -2,19 +2,31 @@
 
 import warnings
 from abc import ABC, abstractmethod
+from collections.abc import Iterator
 from contextlib import contextmanager
 from math import cos, pi, sin, tan
-from typing import TYPE_CHECKING, Protocol
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    ContextManager,
+    Literal,
+    NoReturn,
+    Protocol,
+)
 
 from travertino.colors import Color
 
 import toga
 from toga.colors import BLACK, color as parse_color
 from toga.constants import Baseline, FillRule
-from toga.fonts import SYSTEM, SYSTEM_DEFAULT_FONT_SIZE, Font
+from toga.fonts import (
+    SYSTEM,
+    SYSTEM_DEFAULT_FONT_SIZE,
+    Font,
+)
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
 
 if TYPE_CHECKING:
     from toga.images import ImageT
@@ -54,20 +66,20 @@ class DrawingObject(ABC):
     * :meth:`toga.widgets.canvas.WriteText <Context.write_text>`
     """
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}()"
 
     @abstractmethod
-    def _draw(self, impl, **kwargs): ...
+    def _draw(self, impl: Any, **kwargs: Any) -> None: ...
 
 
 class BeginPath(DrawingObject):
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.begin_path(**kwargs)
 
 
 class ClosePath(DrawingObject):
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.close_path(**kwargs)
 
 
@@ -81,13 +93,13 @@ def __init__(
         self.color = color
         self.fill_rule = fill_rule
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(color={self.color!r}, "
             f"fill_rule={self.fill_rule})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.fill(self.color, self.fill_rule, **kwargs)
 
     @property
@@ -95,7 +107,7 @@ def fill_rule(self) -> FillRule:
         return self._fill_rule
 
     @fill_rule.setter
-    def fill_rule(self, fill_rule: FillRule):
+    def fill_rule(self, fill_rule: FillRule) -> None:
         self._fill_rule = fill_rule
 
     @property
@@ -103,7 +115,7 @@ def color(self) -> Color:
         return self._color
 
     @color.setter
-    def color(self, value: Color | str | None):
+    def color(self, value: Color | str | None) -> None:
         if value is None:
             self._color = parse_color(BLACK)
         else:
@@ -115,10 +127,10 @@ def color(self, value: Color | str | None):
 
     # `context.fill()` used to be a context manager, but is now a primitive.
     # If you try to use the Fill drawing object as a context, raise an exception.
-    def __enter__(self):
+    def __enter__(self) -> NoReturn:
         raise RuntimeError("Context.fill() has been renamed Context.Fill().")
 
-    def __exit__(self):  # pragma: no cover
+    def __exit__(self) -> None:  # pragma: no cover
         # This method is required to make the object a context manager, but as the
         # __enter__ method raises an exception, the __exit__ can't be called.
         pass
@@ -136,13 +148,13 @@ def __init__(
         self.line_width = line_width
         self.line_dash = line_dash
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(color={self.color!r}, "
             f"line_width={self.line_width}, line_dash={self.line_dash!r})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.stroke(self.color, self.line_width, self.line_dash, **kwargs)
 
     @property
@@ -150,7 +162,7 @@ def color(self) -> Color:
         return self._color
 
     @color.setter
-    def color(self, value: Color | str | None):
+    def color(self, value: Color | str | None) -> None:
         if value is None:
             self._color = parse_color(BLACK)
         else:
@@ -161,7 +173,7 @@ def line_width(self) -> float:
         return self._line_width
 
     @line_width.setter
-    def line_width(self, value: float):
+    def line_width(self, value: float) -> None:
         self._line_width = float(value)
 
     @property
@@ -169,7 +181,7 @@ def line_dash(self) -> list[float] | None:
         return self._line_dash
 
     @line_dash.setter
-    def line_dash(self, value: list[float] | None):
+    def line_dash(self, value: list[float] | None) -> None:
         self._line_dash = value
 
     ###########################################################################
@@ -178,10 +190,10 @@ def line_dash(self, value: list[float] | None):
 
     # `context.stroke()` used to be a context manager, but is now a primitive.
     # If you try to use the Stroke drawing object as a context, raise an exception.
-    def __enter__(self):
+    def __enter__(self) -> NoReturn:
         raise RuntimeError("Context.stroke() has been renamed Context.Stroke().")
 
-    def __exit__(self):  # pragma: no cover
+    def __exit__(self) -> None:  # pragma: no cover
         # This method is required to make the object a context manager, but as the
         # __enter__ method raises an exception, the __exit__ can't be called.
         pass
@@ -192,10 +204,10 @@ def __init__(self, x: float, y: float):
         self.x = x
         self.y = y
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(x={self.x}, y={self.y})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.move_to(self.x, self.y, **kwargs)
 
 
@@ -204,10 +216,10 @@ def __init__(self, x: float, y: float):
         self.x = x
         self.y = y
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(x={self.x}, y={self.y})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.line_to(self.x, self.y, **kwargs)
 
 
@@ -222,14 +234,14 @@ def __init__(
         self.x = x
         self.y = y
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(cp1x={self.cp1x}, cp1y={self.cp1y}, "
             f"cp2x={self.cp2x}, cp2y={self.cp2y}, "
             f"x={self.x}, y={self.y})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.bezier_curve_to(
             self.cp1x, self.cp1y, self.cp2x, self.cp2y, self.x, self.y, **kwargs
         )
@@ -242,10 +254,10 @@ def __init__(self, cpx: float, cpy: float, x: float, y: float):
         self.x = x
         self.y = y
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(cpx={self.cpx}, cpy={self.cpy}, x={self.x}, y={self.y})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.quadratic_curve_to(self.cpx, self.cpy, self.x, self.y, **kwargs)
 
 
@@ -266,14 +278,14 @@ def __init__(
         self.endangle = endangle
         self.anticlockwise = anticlockwise
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(x={self.x}, y={self.y}, "
             f"radius={self.radius}, startangle={self.startangle:.3f}, "
             f"endangle={self.endangle:.3f}, anticlockwise={self.anticlockwise})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.arc(
             self.x,
             self.y,
@@ -306,7 +318,7 @@ def __init__(
         self.endangle = endangle
         self.anticlockwise = anticlockwise
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(x={self.x}, y={self.y}, "
             f"radiusx={self.radiusx}, radiusy={self.radiusy}, "
@@ -314,7 +326,7 @@ def __repr__(self):
             f"endangle={self.endangle:.3f}, anticlockwise={self.anticlockwise})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.ellipse(
             self.x,
             self.y,
@@ -335,13 +347,13 @@ def __init__(self, x: float, y: float, width: float, height: float):
         self.width = width
         self.height = height
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(x={self.x}, y={self.y}, "
             f"width={self.width}, height={self.height})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.rect(self.x, self.y, self.width, self.height, **kwargs)
 
 
@@ -360,13 +372,13 @@ def __init__(
         self.font = font
         self.baseline = baseline
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(text={self.text!r}, x={self.x}, y={self.y}, "
             f"font={self.font!r}, baseline={self.baseline})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.write_text(
             str(self.text), self.x, self.y, self.font._impl, self.baseline, **kwargs
         )
@@ -376,7 +388,7 @@ def font(self) -> Font:
         return self._font
 
     @font.setter
-    def font(self, value: Font | None):
+    def font(self, value: Font | None) -> None:
         if value is None:
             self._font = Font(family=SYSTEM, size=SYSTEM_DEFAULT_FONT_SIZE)
         else:
@@ -387,10 +399,10 @@ class Rotate(DrawingObject):
     def __init__(self, radians: float):
         self.radians = radians
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(radians={self.radians:.3f})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.rotate(self.radians, **kwargs)
 
 
@@ -399,10 +411,10 @@ def __init__(self, sx: float, sy: float):
         self.sx = sx
         self.sy = sy
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(sx={self.sx:.3f}, sy={self.sy:.3f})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.scale(self.sx, self.sy, **kwargs)
 
 
@@ -411,15 +423,15 @@ def __init__(self, tx: float, ty: float):
         self.tx = tx
         self.ty = ty
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(tx={self.tx}, ty={self.ty})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.translate(self.tx, self.ty, **kwargs)
 
 
 class ResetTransform(DrawingObject):
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.reset_transform(**kwargs)
 
 
@@ -436,13 +448,13 @@ class Context(DrawingObject):
     or use :any:`Canvas.context` to access the root context of the canvas.
     """
 
-    def __init__(self, canvas: toga.Canvas, **kwargs):
+    def __init__(self, canvas: toga.Canvas, **kwargs: Any):
         # kwargs used to support multiple inheritance
         super().__init__(**kwargs)
         self._canvas = canvas
-        self.drawing_objects = []
+        self.drawing_objects: list[DrawingObject] = []
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.push_context(**kwargs)
         for obj in self.drawing_objects:
             obj._draw(impl, **kwargs)
@@ -457,7 +469,7 @@ def canvas(self) -> Canvas:
         """The canvas that is associated with this drawing context."""
         return self._canvas
 
-    def redraw(self):
+    def redraw(self) -> None:
         """Calls :any:`Canvas.redraw` on the parent Canvas."""
         self.canvas.redraw()
 
@@ -473,7 +485,7 @@ def __getitem__(self, index: int) -> DrawingObject:
         """Returns the drawing object at the given index."""
         return self.drawing_objects[index]
 
-    def append(self, obj: DrawingObject):
+    def append(self, obj: DrawingObject) -> None:
         """Append a drawing object to the context.
 
         :param obj: The drawing object to add to the context.
@@ -481,7 +493,7 @@ def append(self, obj: DrawingObject):
         self.drawing_objects.append(obj)
         self.redraw()
 
-    def insert(self, index: int, obj: DrawingObject):
+    def insert(self, index: int, obj: DrawingObject) -> None:
         """Insert a drawing object into the context at a specific index.
 
         :param index: The index at which the drawing object should be inserted.
@@ -490,7 +502,7 @@ def insert(self, index: int, obj: DrawingObject):
         self.drawing_objects.insert(index, obj)
         self.redraw()
 
-    def remove(self, obj: DrawingObject):
+    def remove(self, obj: DrawingObject) -> None:
         """Remove a drawing object from the context.
 
         :param obj: The drawing object to remove.
@@ -498,7 +510,7 @@ def remove(self, obj: DrawingObject):
         self.drawing_objects.remove(obj)
         self.redraw()
 
-    def clear(self):
+    def clear(self) -> None:
         """Remove all drawing objects from the context."""
         self.drawing_objects.clear()
         self.redraw()
@@ -507,7 +519,7 @@ def clear(self):
     # Path manipulation
     ###########################################################################
 
-    def begin_path(self):
+    def begin_path(self) -> BeginPath:
         """Start a new path in the canvas context.
 
         :returns: The ``BeginPath`` :any:`DrawingObject` for the operation.
@@ -516,7 +528,7 @@ def begin_path(self):
         self.append(begin_path)
         return begin_path
 
-    def close_path(self):
+    def close_path(self) -> ClosePath:
         """Close the current path in the canvas context.
 
         This closes the current path as a simple drawing operation. It should be paired
@@ -530,7 +542,7 @@ def close_path(self):
         self.append(close_path)
         return close_path
 
-    def move_to(self, x: float, y: float):
+    def move_to(self, x: float, y: float) -> MoveTo:
         """Moves the current point of the canvas context without drawing.
 
         :param x: The x coordinate of the new current point.
@@ -541,7 +553,7 @@ def move_to(self, x: float, y: float):
         self.append(move_to)
         return move_to
 
-    def line_to(self, x: float, y: float):
+    def line_to(self, x: float, y: float) -> LineTo:
         """Draw a line segment ending at a point in the canvas context.
 
         :param x: The x coordinate for the end point of the line segment.
@@ -560,7 +572,7 @@ def bezier_curve_to(
         cp2y: float,
         x: float,
         y: float,
-    ):
+    ) -> BezierCurveTo:
         """Draw a Bézier curve in the canvas context.
 
         A Bézier curve requires three points. The first two are control points; the
@@ -580,7 +592,13 @@ def bezier_curve_to(
         self.append(bezier_curve_to)
         return bezier_curve_to
 
-    def quadratic_curve_to(self, cpx: float, cpy: float, x: float, y: float):
+    def quadratic_curve_to(
+        self,
+        cpx: float,
+        cpy: float,
+        x: float,
+        y: float,
+    ) -> QuadraticCurveTo:
         """Draw a quadratic curve in the canvas context.
 
         A quadratic curve requires two points. The first point is a control point; the
@@ -608,7 +626,7 @@ def arc(
         startangle: float = 0.0,
         endangle: float = 2 * pi,
         anticlockwise: bool = False,
-    ):
+    ) -> Arc:
         """Draw a circular arc in the canvas context.
 
         A full circle will be drawn by default; an arc can be drawn by specifying a
@@ -638,7 +656,7 @@ def ellipse(
         startangle: float = 0.0,
         endangle: float = 2 * pi,
         anticlockwise: bool = False,
-    ):
+    ) -> Ellipse:
         """Draw an elliptical arc in the canvas context.
 
         A full ellipse will be drawn by default; an arc can be drawn by specifying a
@@ -671,7 +689,7 @@ def ellipse(
         self.append(ellipse)
         return ellipse
 
-    def rect(self, x: float, y: float, width: float, height: float):
+    def rect(self, x: float, y: float, width: float, height: float) -> Rect:
         """Draw a rectangle in the canvas context.
 
         :param x: The horizontal coordinate of the left of the rectangle.
@@ -688,8 +706,8 @@ def fill(
         self,
         color: str = BLACK,
         fill_rule: FillRule = FillRule.NONZERO,
-        preserve=None,  # DEPRECATED
-    ):
+        preserve: None = None,  # DEPRECATED
+    ) -> Fill:
         """Fill the current path.
 
         The fill can use either the `Non-Zero
@@ -717,7 +735,7 @@ def stroke(
         color: str = BLACK,
         line_width: float = 2.0,
         line_dash: list[float] | None = None,
-    ):
+    ) -> Stroke:
         """Draw the current path as a stroke.
 
         :param color: The color for the stroke.
@@ -741,7 +759,7 @@ def write_text(
         y: float = 0.0,
         font: Font | None = None,
         baseline: Baseline = Baseline.ALPHABETIC,
-    ):
+    ) -> WriteText:
         """Write text at a given position in the canvas context.
 
         Drawing text is effectively a series of path operations, so the text will have
@@ -762,7 +780,7 @@ def write_text(
     ###########################################################################
     # Transformations
     ###########################################################################
-    def rotate(self, radians: float):
+    def rotate(self, radians: float) -> Rotate:
         """Add a rotation to the canvas context.
 
         :param radians: The angle to rotate clockwise in radians.
@@ -772,7 +790,7 @@ def rotate(self, radians: float):
         self.append(rotate)
         return rotate
 
-    def scale(self, sx: float, sy: float):
+    def scale(self, sx: float, sy: float) -> Scale:
         """Add a scaling transformation to the canvas context.
 
         :param sx: Scale factor for the X dimension. A negative value flips the
@@ -785,7 +803,7 @@ def scale(self, sx: float, sy: float):
         self.append(scale)
         return scale
 
-    def translate(self, tx: float, ty: float):
+    def translate(self, tx: float, ty: float) -> Translate:
         """Add a translation to the canvas context.
 
         :param tx: Translation for the X dimension.
@@ -796,7 +814,7 @@ def translate(self, tx: float, ty: float):
         self.append(translate)
         return translate
 
-    def reset_transform(self):
+    def reset_transform(self) -> ResetTransform:
         """Reset all transformations in the canvas context.
 
         :returns: A ``ResetTransform`` :any:`DrawingObject`.
@@ -810,7 +828,7 @@ def reset_transform(self):
     ###########################################################################
 
     @contextmanager
-    def Context(self):
+    def Context(self) -> Iterator[Context]:
         """Construct and yield a new sub-:class:`~toga.widgets.canvas.Context` within
         this context.
 
@@ -822,7 +840,11 @@ def Context(self):
         self.redraw()
 
     @contextmanager
-    def ClosedPath(self, x: float | None = None, y: float | None = None):
+    def ClosedPath(
+        self,
+        x: float | None = None,
+        y: float | None = None,
+    ) -> Iterator[ClosedPathContext]:
         """Construct and yield a new :class:`~toga.widgets.canvas.ClosedPath`
         sub-context that will draw a closed path, starting from an origin.
 
@@ -846,7 +868,7 @@ def Fill(
         y: float | None = None,
         color: str = BLACK,
         fill_rule: FillRule = FillRule.NONZERO,
-    ):
+    ) -> Iterator[FillContext]:
         """Construct and yield a new :class:`~toga.widgets.canvas.Fill` sub-context
         within this context.
 
@@ -886,7 +908,7 @@ def Stroke(
         color: str = BLACK,
         line_width: float = 2.0,
         line_dash: list[float] | None = None,
-    ):
+    ) -> Iterator[StrokeContext]:
         """Construct and yield a new :class:`~toga.widgets.canvas.Stroke` sub-context
         within this context.
 
@@ -924,7 +946,7 @@ def Stroke(
     # 2023-07 Backwards incompatibility
     ###########################################################################
 
-    def new_path(self):
+    def new_path(self) -> BeginPath:
         """**DEPRECATED** - Use :meth:`~toga.widgets.canvas.Context.begin_path`."""
         warnings.warn(
             "Context.new_path() has been renamed Context.begin_path()",
@@ -976,10 +998,10 @@ def __init__(
         self.x = x
         self.y = y
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"{self.__class__.__name__}(x={self.x}, y={self.y})"
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         """Used by parent to draw all objects that are part of the context."""
         impl.push_context(**kwargs)
         impl.begin_path(**kwargs)
@@ -1029,13 +1051,13 @@ def __init__(
         self.color = color
         self.fill_rule = fill_rule
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(x={self.x}, y={self.y}, "
             f"color={self.color!r}, fill_rule={self.fill_rule})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.push_context(**kwargs)
         impl.begin_path(**kwargs)
         if self.x is not None and self.y is not None:
@@ -1062,7 +1084,7 @@ def color(self) -> Color:
         return self._color
 
     @color.setter
-    def color(self, value: Color | str | None):
+    def color(self, value: Color | str | None) -> None:
         if value is None:
             self._color = parse_color(BLACK)
         else:
@@ -1102,13 +1124,13 @@ def __init__(
         self.line_width = line_width
         self.line_dash = line_dash
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return (
             f"{self.__class__.__name__}(x={self.x}, y={self.y}, color={self.color!r}, "
             f"line_width={self.line_width}, line_dash={self.line_dash!r})"
         )
 
-    def _draw(self, impl, **kwargs):
+    def _draw(self, impl: Any, **kwargs: Any) -> None:
         impl.push_context(**kwargs)
         impl.begin_path(**kwargs)
 
@@ -1139,7 +1161,7 @@ def color(self) -> Color:
         return self._color
 
     @color.setter
-    def color(self, value):
+    def color(self, value: object) -> None:
         if value is None:
             self._color = parse_color(BLACK)
         else:
@@ -1152,7 +1174,7 @@ def color(self, value):
 
 
 class OnTouchHandler(Protocol):
-    def __call__(self, widget: Canvas, x: int, y: int, **kwargs):
+    def __call__(self, widget: Canvas, x: int, y: int, /, **kwargs: Any) -> object:
         """A handler that will be invoked when a :any:`Canvas` is touched with a finger
         or mouse.
 
@@ -1161,11 +1183,12 @@ def __call__(self, widget: Canvas, x: int, y: int, **kwargs):
         :param y: Y coordinate, relative to the top edge of the canvas.
         :param kwargs: Ensures compatibility with arguments added in future versions.
         """
-        ...
 
 
 class OnResizeHandler(Protocol):
-    def __call__(self, widget: Canvas, width: int, height: int, **kwargs):
+    def __call__(
+        self, widget: Canvas, /, width: int, height: int, **kwargs: Any
+    ) -> object:
         """A handler that will be invoked when a :any:`Canvas` is resized.
 
         :param widget: The canvas that was resized.
@@ -1173,7 +1196,6 @@ def __call__(self, widget: Canvas, width: int, height: int, **kwargs):
         :param height: The new height.
         :param kwargs: Ensures compatibility with arguments added in future versions.
         """
-        ...
 
 
 #######################################################################################
@@ -1187,16 +1209,16 @@ class Canvas(Widget):
 
     def __init__(
         self,
-        id=None,
-        style=None,
-        on_resize: OnResizeHandler = None,
-        on_press: OnTouchHandler = None,
-        on_activate: OnTouchHandler = None,
-        on_release: OnTouchHandler = None,
-        on_drag: OnTouchHandler = None,
-        on_alt_press: OnTouchHandler = None,
-        on_alt_release: OnTouchHandler = None,
-        on_alt_drag: OnTouchHandler = None,
+        id: str | None = None,
+        style: StyleT | None = None,
+        on_resize: OnResizeHandler | None = None,
+        on_press: OnTouchHandler | None = None,
+        on_activate: OnTouchHandler | None = None,
+        on_release: OnTouchHandler | None = None,
+        on_drag: OnTouchHandler | None = None,
+        on_alt_press: OnTouchHandler | None = None,
+        on_alt_release: OnTouchHandler | None = None,
+        on_alt_drag: OnTouchHandler | None = None,
     ):
         """Create a new Canvas widget.
 
@@ -1233,7 +1255,7 @@ def __init__(
         self.on_alt_drag = on_alt_drag
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
         ScrollContainer widgets cannot be disabled; this property will always return
         True; any attempt to modify it will be ignored.
@@ -1241,11 +1263,11 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; ScrollContainer cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; ScrollContainer cannot accept input focus."""
         pass
 
     @property
@@ -1253,7 +1275,7 @@ def context(self) -> Context:
         """The root context for the canvas."""
         return self._context
 
-    def redraw(self):
+    def redraw(self) -> None:
         """Redraw the Canvas.
 
         The Canvas will be automatically redrawn after adding or removing a drawing
@@ -1262,7 +1284,7 @@ def redraw(self):
         """
         self._impl.redraw()
 
-    def Context(self):
+    def Context(self) -> ContextManager[Context]:
         """Construct and yield a new sub-:class:`~toga.widgets.canvas.Context` within
         the root context of this Canvas.
 
@@ -1270,7 +1292,11 @@ def Context(self):
         """
         return self.context.Context()
 
-    def ClosedPath(self, x: float | None = None, y: float | None = None):
+    def ClosedPath(
+        self,
+        x: float | None = None,
+        y: float | None = None,
+    ) -> ContextManager[ClosedPathContext]:
         """Construct and yield a new :class:`~toga.widgets.canvas.ClosedPathContext` context in
         the root context of this canvas.
 
@@ -1286,7 +1312,7 @@ def Fill(
         y: float | None = None,
         color: Color | str | None = BLACK,
         fill_rule: FillRule = FillRule.NONZERO,
-    ):
+    ) -> ContextManager[FillContext]:
         """Construct and yield a new :class:`~toga.widgets.canvas.FillContext` in the
         root context of this canvas.
 
@@ -1312,7 +1338,7 @@ def Stroke(
         color: Color | str | None = BLACK,
         line_width: float = 2.0,
         line_dash: list[float] | None = None,
-    ):
+    ) -> ContextManager[StrokeContext]:
         """Construct and yield a new :class:`~toga.widgets.canvas.StrokeContext` in the
         root context of this canvas.
 
@@ -1335,7 +1361,7 @@ def on_resize(self) -> OnResizeHandler:
         return self._on_resize
 
     @on_resize.setter
-    def on_resize(self, handler: OnResizeHandler):
+    def on_resize(self, handler: OnResizeHandler) -> None:
         self._on_resize = wrapped_handler(self, handler)
 
     @property
@@ -1345,7 +1371,7 @@ def on_press(self) -> OnTouchHandler:
         return self._on_press
 
     @on_press.setter
-    def on_press(self, handler: OnTouchHandler):
+    def on_press(self, handler: OnTouchHandler) -> None:
         self._on_press = wrapped_handler(self, handler)
 
     @property
@@ -1358,7 +1384,7 @@ def on_activate(self) -> OnTouchHandler:
         return self._on_activate
 
     @on_activate.setter
-    def on_activate(self, handler: OnTouchHandler):
+    def on_activate(self, handler: OnTouchHandler) -> None:
         self._on_activate = wrapped_handler(self, handler)
 
     @property
@@ -1367,7 +1393,7 @@ def on_release(self) -> OnTouchHandler:
         return self._on_release
 
     @on_release.setter
-    def on_release(self, handler: OnTouchHandler):
+    def on_release(self, handler: OnTouchHandler) -> None:
         self._on_release = wrapped_handler(self, handler)
 
     @property
@@ -1376,7 +1402,7 @@ def on_drag(self) -> OnTouchHandler:
         return self._on_drag
 
     @on_drag.setter
-    def on_drag(self, handler: OnTouchHandler):
+    def on_drag(self, handler: OnTouchHandler) -> None:
         self._on_drag = wrapped_handler(self, handler)
 
     @property
@@ -1390,7 +1416,7 @@ def on_alt_press(self) -> OnTouchHandler:
         return self._on_alt_press
 
     @on_alt_press.setter
-    def on_alt_press(self, handler: OnTouchHandler):
+    def on_alt_press(self, handler: OnTouchHandler) -> None:
         self._on_alt_press = wrapped_handler(self, handler)
 
     @property
@@ -1402,7 +1428,7 @@ def on_alt_release(self) -> OnTouchHandler:
         return self._on_alt_release
 
     @on_alt_release.setter
-    def on_alt_release(self, handler: OnTouchHandler):
+    def on_alt_release(self, handler: OnTouchHandler) -> None:
         self._on_alt_release = wrapped_handler(self, handler)
 
     @property
@@ -1414,7 +1440,7 @@ def on_alt_drag(self) -> OnTouchHandler:
         return self._on_alt_drag
 
     @on_alt_drag.setter
-    def on_alt_drag(self, handler: OnTouchHandler):
+    def on_alt_drag(self, handler: OnTouchHandler) -> None:
         self._on_alt_drag = wrapped_handler(self, handler)
 
     ###########################################################################
@@ -1425,7 +1451,7 @@ def measure_text(
         self,
         text: str,
         font: Font | None = None,
-        tight=None,  # DEPRECATED
+        tight: None = None,  # DEPRECATED
     ) -> tuple[float, float]:
         """Measure the size at which :meth:`~.Context.write_text` would render some text.
 
@@ -1639,7 +1665,7 @@ def stroke(
         return self.Stroke(color=color, line_width=line_width, line_dash=line_dash)
 
 
-def sweepangle(startangle, endangle, anticlockwise):
+def sweepangle(startangle: float, endangle: float, anticlockwise: bool) -> float:
     """Returns an arc length in the range [-2 * pi, 2 * pi], where positive numbers are
     clockwise. Based on the "ellipse method steps" in the HTML spec."""
 
@@ -1664,7 +1690,7 @@ def sweepangle(startangle, endangle, anticlockwise):
 
 
 # Based on https://stackoverflow.com/a/30279817
-def arc_to_bezier(sweepangle):
+def arc_to_bezier(sweepangle: float) -> list[tuple[float, float]]:
     """Approximates an arc of a unit circle as a sequence of Bezier segments.
 
     :param sweepangle: Length of the arc in radians, where positive numbers are
@@ -1707,7 +1733,7 @@ def arc_to_bezier(sweepangle):
     return result
 
 
-def transform(x, y, matrix):
+def transform(x: float, y: float, matrix: list[int]) -> tuple[float, float]:
     return (
         x * matrix[0] + y * matrix[1],
         x * matrix[2] + y * matrix[3],
diff --git a/core/src/toga/widgets/dateinput.py b/core/src/toga/widgets/dateinput.py
index 53b050816b..7408f38ae2 100644
--- a/core/src/toga/widgets/dateinput.py
+++ b/core/src/toga/widgets/dateinput.py
@@ -2,10 +2,12 @@
 
 import datetime
 import warnings
+from typing import Any, Protocol
 
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
 
 # This accommodates the ranges of all existing implementations:
 #  * datetime.date: 1 - 9999
@@ -15,17 +17,26 @@
 MAX_DATE = datetime.date(8999, 12, 31)
 
 
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: DateInput, /, **kwargs: Any) -> object:
+        """A handler that will be invoked when a change occurs.
+
+        :param widget: The DateInput that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
 class DateInput(Widget):
     _MIN_WIDTH = 200
 
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         value: datetime.date | None = None,
         min: datetime.date | None = None,
         max: datetime.date | None = None,
-        on_change: callable | None = None,
+        on_change: toga.widgets.dateinput.OnChangeHandler | None = None,
     ):
         """Create a new DateInput widget.
 
@@ -60,7 +71,18 @@ def value(self) -> datetime.date:
         """
         return self._impl.get_value()
 
-    def _convert_date(self, value, *, check_range):
+    @value.setter
+    def value(self, value: object) -> None:
+        value = self._convert_date(value, check_range=False)
+
+        if value < self.min:
+            value = self.min
+        elif value > self.max:
+            value = self.max
+
+        self._impl.set_value(value)
+
+    def _convert_date(self, value: object, *, check_range: bool) -> datetime.date:
         if value is None:
             value = datetime.date.today()
         elif isinstance(value, datetime.datetime):
@@ -82,17 +104,6 @@ def _convert_date(self, value, *, check_range):
 
         return value
 
-    @value.setter
-    def value(self, value):
-        value = self._convert_date(value, check_range=False)
-
-        if value < self.min:
-            value = self.min
-        elif value > self.max:
-            value = self.max
-
-        self._impl.set_value(value)
-
     @property
     def min(self) -> datetime.date:
         """The minimum allowable date (inclusive). A value of ``None`` will be converted
@@ -106,7 +117,7 @@ def min(self) -> datetime.date:
         return self._impl.get_min_date()
 
     @min.setter
-    def min(self, value):
+    def min(self, value: object) -> None:
         if value is None:
             min = MIN_DATE
         else:
@@ -131,7 +142,7 @@ def max(self) -> datetime.date:
         return self._impl.get_max_date()
 
     @max.setter
-    def max(self, value):
+    def max(self, value: object) -> None:
         if value is None:
             max = MAX_DATE
         else:
@@ -144,18 +155,18 @@ def max(self, value):
             self.value = max
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """The handler to invoke when the date value changes."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.dateinput.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
 
 # 2023-05: Backwards compatibility
 class DatePicker(DateInput):
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args: Any, **kwargs: Any):
         warnings.warn("DatePicker has been renamed DateInput.", DeprecationWarning)
 
         for old_name, new_name in [
@@ -176,28 +187,28 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
     @property
-    def min_date(self):
+    def min_date(self) -> datetime.date:
         warnings.warn(
             "DatePicker.min_date has been renamed DateInput.min", DeprecationWarning
         )
         return self.min
 
     @min_date.setter
-    def min_date(self, value):
+    def min_date(self, value: object) -> None:
         warnings.warn(
             "DatePicker.min_date has been renamed DateInput.min", DeprecationWarning
         )
         self.min = value
 
     @property
-    def max_date(self):
+    def max_date(self) -> datetime.date:
         warnings.warn(
             "DatePicker.max_date has been renamed DateInput.max", DeprecationWarning
         )
         return self.max
 
     @max_date.setter
-    def max_date(self, value):
+    def max_date(self, value: object) -> None:
         warnings.warn(
             "DatePicker.max_date has been renamed DateInput.max", DeprecationWarning
         )
diff --git a/core/src/toga/widgets/detailedlist.py b/core/src/toga/widgets/detailedlist.py
index 2b4d818f9a..a5ec7937e0 100644
--- a/core/src/toga/widgets/detailedlist.py
+++ b/core/src/toga/widgets/detailedlist.py
@@ -1,29 +1,71 @@
 from __future__ import annotations
 
 import warnings
-from typing import Any
+from collections.abc import Iterable
+from typing import Any, Literal, Protocol, TypeVar
 
+import toga
 from toga.handlers import wrapped_handler
 from toga.sources import ListSource, Row, Source
 
-from .base import Widget
+from .base import StyleT, Widget
+
+SourceT = TypeVar("SourceT", bound=Source)
+
+
+class OnPrimaryActionHandler(Protocol):
+    def __call__(self, widget: DetailedList, row: Any, /, **kwargs: Any) -> object:
+        """A handler to invoke for the primary action.
+
+        :param widget: The DetailedList that was invoked.
+        :param row: The current row for the detailed list.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnSecondaryActionHandler(Protocol):
+    def __call__(self, widget: DetailedList, row: Any, /, **kwargs: Any) -> object:
+        """A handler to invoke for the secondary action.
+
+        :param widget: The DetailedList that was invoked.
+        :param row: The current row for the detailed list.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnRefreshHandler(Protocol):
+    def __call__(self, widget: DetailedList, /, **kwargs: Any) -> object:
+        """A handler to invoke when the detailed list is refreshed.
+
+        :param widget: The DetailedList that was refreshed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnSelectHandler(Protocol):
+    def __call__(self, widget: DetailedList, /, **kwargs: Any) -> object:
+        """A handler to invoke when the detailed list is selected.
+
+        :param widget: The DetailedList that was selected.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class DetailedList(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
-        data: Any = None,
+        id: str | None = None,
+        style: StyleT | None = None,
+        data: SourceT | Iterable | None = None,
         accessors: tuple[str, str, str] = ("title", "subtitle", "icon"),
         missing_value: str = "",
         primary_action: str | None = "Delete",
-        on_primary_action: callable = None,
+        on_primary_action: OnPrimaryActionHandler | None = None,
         secondary_action: str | None = "Action",
-        on_secondary_action: callable = None,
-        on_refresh: callable = None,
-        on_select: callable = None,
-        on_delete: callable = None,  # DEPRECATED
+        on_secondary_action: OnSecondaryActionHandler | None = None,
+        on_refresh: OnRefreshHandler | None = None,
+        on_select: toga.widgets.detailedlist.OnSelectHandler | None = None,
+        on_delete: None = None,  # DEPRECATED
     ):
         """Create a new DetailedList widget.
 
@@ -63,12 +105,13 @@ def __init__(
 
         # Prime the attributes and handlers that need to exist when the widget is created.
         self._accessors = accessors
+        self._missing_value = missing_value
         self._primary_action = primary_action
         self._secondary_action = secondary_action
-        self._missing_value = missing_value
-        self._data = None
         self.on_select = None
 
+        self._data: SourceT | ListSource = None
+
         self._impl = self.factory.DetailedList(interface=self)
 
         self.data = data
@@ -78,7 +121,7 @@ def __init__(
         self.on_select = on_select
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
         DetailedList widgets cannot be disabled; this property will always return True; any
         attempt to modify it will be ignored.
@@ -86,15 +129,15 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; DetailedList cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; DetailedList cannot accept input focus."""
         pass
 
     @property
-    def data(self) -> ListSource:
+    def data(self) -> SourceT | ListSource:
         """The data to display in the table.
 
         When setting this property:
@@ -110,7 +153,7 @@ def data(self) -> ListSource:
         return self._data
 
     @data.setter
-    def data(self, data: Any):
+    def data(self, data: SourceT | Iterable | None) -> None:
         if data is None:
             self._data = ListSource(data=[], accessors=self.accessors)
         elif isinstance(data, Source):
@@ -121,11 +164,11 @@ def data(self, data: Any):
         self._data.add_listener(self._impl)
         self._impl.change_source(source=self._data)
 
-    def scroll_to_top(self):
+    def scroll_to_top(self) -> None:
         """Scroll the view so that the top of the list (first row) is visible."""
         self.scroll_to_row(0)
 
-    def scroll_to_row(self, row: int):
+    def scroll_to_row(self, row: int) -> None:
         """Scroll the view so that the specified row index is visible.
 
         :param row: The index of the row to make visible. Negative values refer to the
@@ -137,12 +180,12 @@ def scroll_to_row(self, row: int):
             else:
                 self._impl.scroll_to_row(max(len(self.data) + row, 0))
 
-    def scroll_to_bottom(self):
+    def scroll_to_bottom(self) -> None:
         """Scroll the view so that the bottom of the list (last row) is visible."""
         self.scroll_to_row(-1)
 
     @property
-    def accessors(self) -> list[str]:
+    def accessors(self) -> tuple[str, str, str]:
         """The accessors used to populate the list (read-only)"""
         return self._accessors
 
@@ -165,7 +208,7 @@ def selection(self) -> Row | None:
             return None
 
     @property
-    def on_primary_action(self) -> callable:
+    def on_primary_action(self) -> OnPrimaryActionHandler:
         """The handler to invoke when the user performs the primary action on a row of
         the DetailedList.
 
@@ -178,12 +221,12 @@ def on_primary_action(self) -> callable:
         return self._on_primary_action
 
     @on_primary_action.setter
-    def on_primary_action(self, handler: callable):
+    def on_primary_action(self, handler: OnPrimaryActionHandler) -> None:
         self._on_primary_action = wrapped_handler(self, handler)
         self._impl.set_primary_action_enabled(handler is not None)
 
     @property
-    def on_secondary_action(self) -> callable:
+    def on_secondary_action(self) -> OnSecondaryActionHandler:
         """The handler to invoke when the user performs the secondary action on a row of
         the DetailedList.
 
@@ -196,12 +239,12 @@ def on_secondary_action(self) -> callable:
         return self._on_secondary_action
 
     @on_secondary_action.setter
-    def on_secondary_action(self, handler: callable):
+    def on_secondary_action(self, handler: OnSecondaryActionHandler) -> None:
         self._on_secondary_action = wrapped_handler(self, handler)
         self._impl.set_secondary_action_enabled(handler is not None)
 
     @property
-    def on_refresh(self) -> callable:
+    def on_refresh(self) -> OnRefreshHandler:
         """The callback function to invoke when the user performs a refresh action
         (usually "pull down") on the DetailedList.
 
@@ -211,19 +254,19 @@ def on_refresh(self) -> callable:
         return self._on_refresh
 
     @on_refresh.setter
-    def on_refresh(self, handler: callable):
+    def on_refresh(self, handler: OnRefreshHandler) -> None:
         self._on_refresh = wrapped_handler(
             self, handler, cleanup=self._impl.after_on_refresh
         )
         self._impl.set_refresh_enabled(handler is not None)
 
     @property
-    def on_select(self) -> callable:
+    def on_select(self) -> OnSelectHandler:
         """The callback function that is invoked when a row of the DetailedList is selected."""
         return self._on_select
 
     @on_select.setter
-    def on_select(self, handler: callable):
+    def on_select(self, handler: toga.widgets.detailedlist.OnSelectHandler) -> None:
         self._on_select = wrapped_handler(self, handler)
 
     ######################################################################
@@ -231,7 +274,7 @@ def on_select(self, handler: callable):
     ######################################################################
 
     @property
-    def on_delete(self):
+    def on_delete(self) -> OnPrimaryActionHandler:
         """**DEPRECATED**; Use :any:`on_primary_action`"""
         warnings.warn(
             "DetailedList.on_delete has been renamed DetailedList.on_primary_action.",
@@ -240,7 +283,7 @@ def on_delete(self):
         return self.on_primary_action
 
     @on_delete.setter
-    def on_delete(self, handler):
+    def on_delete(self, handler: OnPrimaryActionHandler) -> None:
         warnings.warn(
             "DetailedList.on_delete has been renamed DetailedList.on_primary_action.",
             DeprecationWarning,
diff --git a/core/src/toga/widgets/divider.py b/core/src/toga/widgets/divider.py
index c9103974c1..4b7edf62c1 100644
--- a/core/src/toga/widgets/divider.py
+++ b/core/src/toga/widgets/divider.py
@@ -1,8 +1,10 @@
 from __future__ import annotations
 
+from typing import Literal
+
 from toga.constants import Direction
 
-from .base import Widget
+from .base import StyleT, Widget
 
 
 class Divider(Widget):
@@ -11,8 +13,8 @@ class Divider(Widget):
 
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         direction: Direction = HORIZONTAL,
     ):
         """Create a new divider line.
@@ -32,7 +34,7 @@ def __init__(
         self.direction = direction
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
 
         Divider widgets cannot be disabled; this property will always return True; any
@@ -41,11 +43,11 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; Divider cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; Divider cannot accept input focus."""
         pass
 
     @property
@@ -54,6 +56,6 @@ def direction(self) -> Direction:
         return self._impl.get_direction()
 
     @direction.setter
-    def direction(self, value):
+    def direction(self, value: object) -> None:
         self._impl.set_direction(value)
         self.refresh()
diff --git a/core/src/toga/widgets/imageview.py b/core/src/toga/widgets/imageview.py
index e269a2ce00..663b3d1071 100644
--- a/core/src/toga/widgets/imageview.py
+++ b/core/src/toga/widgets/imageview.py
@@ -1,18 +1,22 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Literal
 
 from travertino.size import at_least
 
 import toga
 from toga.style.pack import NONE
-from toga.widgets.base import Widget
+from toga.widgets.base import StyleT, Widget
 
 if TYPE_CHECKING:
-    from toga.images import ImageContent, ImageT
+    from toga.images import ImageContentT, ImageT
 
 
-def rehint_imageview(image, style, scale=1):
+def rehint_imageview(
+    image: toga.Image,
+    style: StyleT,
+    scale: int = 1,
+) -> tuple[int, int, float | None]:
     """Compute the size hints for an ImageView based on the image.
 
     This logic is common across all backends, so it's shared here.
@@ -63,20 +67,18 @@ def rehint_imageview(image, style, scale=1):
     return width, height, aspect_ratio
 
 
-# Note: remove PIL type annotation when plugin system is implemented for image format
-# registration; replace with ImageT?
 class ImageView(Widget):
     def __init__(
         self,
-        image: ImageContent | None = None,
-        id=None,
-        style=None,
+        image: ImageContentT | None = None,
+        id: str | None = None,
+        style: StyleT | None = None,
     ):
         """
         Create a new image view.
 
         :param image: The image to display. Can be any valid :any:`image content
-            <ImageContent>` type; or :any:`None` to display no image.
+            <ImageContentT>` type; or :any:`None` to display no image.
         :param id: The ID for the widget.
         :param style: A style object. If no style is provided, a default style will be
             applied to the widget.
@@ -88,7 +90,7 @@ def __init__(
         self.image = image
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
 
         ImageView widgets cannot be disabled; this property will always return True; any
@@ -97,11 +99,11 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; ImageView cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; ImageView cannot accept input focus."""
         pass
 
     @property
@@ -109,12 +111,12 @@ def image(self) -> toga.Image | None:
         """The image to display.
 
         When setting an image, you can provide any valid :any:`image content
-        <ImageContent>` type; or :any:`None` to clear the image view.
+        <ImageContentT>` type; or :any:`None` to clear the image view.
         """
         return self._image
 
     @image.setter
-    def image(self, image):
+    def image(self, image: ImageContentT) -> None:
         if isinstance(image, toga.Image):
             self._image = image
         elif image is None:
diff --git a/core/src/toga/widgets/label.py b/core/src/toga/widgets/label.py
index 88a5fd1fad..83fb1ba7a6 100644
--- a/core/src/toga/widgets/label.py
+++ b/core/src/toga/widgets/label.py
@@ -1,14 +1,14 @@
 from __future__ import annotations
 
-from .base import Widget
+from .base import StyleT, Widget
 
 
 class Label(Widget):
     def __init__(
         self,
         text: str,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
     ):
         """Create a new text label.
 
@@ -24,8 +24,8 @@ def __init__(
 
         self.text = text
 
-    def focus(self):
-        "No-op; Label cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; Label cannot accept input focus."""
         pass
 
     @property
@@ -39,7 +39,7 @@ def text(self) -> str:
         return self._impl.get_text()
 
     @text.setter
-    def text(self, value):
+    def text(self, value: object) -> None:
         if value is None or value == "\u200B":
             text = ""
         else:
diff --git a/core/src/toga/widgets/mapview.py b/core/src/toga/widgets/mapview.py
index 5ebda6afca..dd69651b07 100644
--- a/core/src/toga/widgets/mapview.py
+++ b/core/src/toga/widgets/mapview.py
@@ -1,11 +1,12 @@
 from __future__ import annotations
 
+from collections.abc import Iterable, Iterator
 from typing import Any, Protocol
 
 import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
 
 
 class MapPin:
@@ -27,10 +28,10 @@ def __init__(
         self._subtitle = subtitle
 
         # A pin isn't tied to a map at time of creation.
-        self.interface = None
+        self.interface: MapView | None = None
         self._native = None
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         if self.subtitle:
             label = f"; {self.title} - {self.subtitle}"
         else:
@@ -40,7 +41,7 @@ def __repr__(self):
 
     @property
     def location(self) -> toga.LatLng:
-        "The (latitude, longitude) where the pin is located."
+        """The (latitude, longitude) where the pin is located."""
         return self._location
 
     @location.setter
@@ -62,7 +63,7 @@ def title(self, title: str) -> None:
 
     @property
     def subtitle(self) -> str | None:
-        "The subtitle of the pin."
+        """The subtitle of the pin."""
         return self._subtitle
 
     @subtitle.setter
@@ -75,26 +76,26 @@ def subtitle(self, subtitle: str | None) -> None:
 
 
 class MapPinSet:
-    def __init__(self, interface, pins):
+    def __init__(self, interface: MapView, pins: Iterable[MapPin] | None):
         self.interface = interface
-        self._pins = set()
+        self._pins: set[MapPin] = set()
 
-        if pins:
+        if pins is not None:
             for item in pins:
                 self.add(item)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"<MapPinSet ({len(self)} pins)>"
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator[MapPin]:
         """Return an iterator over the pins on the map."""
         return iter(self._pins)
 
-    def __len__(self):
+    def __len__(self) -> int:
         """Return the number of pins being displayed."""
         return len(self._pins)
 
-    def add(self, pin):
+    def add(self, pin: MapPin) -> None:
         """Add a new pin to the map.
 
         :param pin: The :any:`toga.MapPin` instance to add.
@@ -103,7 +104,7 @@ def add(self, pin):
         self._pins.add(pin)
         self.interface._impl.add_pin(pin)
 
-    def remove(self, pin):
+    def remove(self, pin: MapPin) -> None:
         """Remove a pin from the map.
 
         :param pin: The  :any:`toga.MapPin` instance to remove.
@@ -112,7 +113,7 @@ def remove(self, pin):
         self._pins.remove(pin)
         pin.interface = None
 
-    def clear(self):
+    def clear(self) -> None:
         """Remove all pins from the map."""
         for pin in self._pins:
             self.interface._impl.remove_pin(pin)
@@ -120,24 +121,23 @@ def clear(self):
 
 
 class OnSelectHandler(Protocol):
-    def __call__(self, widget: MapView, *, pin: MapPin, **kwargs: Any) -> None:
+    def __call__(self, widget: MapView, /, *, pin: MapPin, **kwargs: Any) -> object:
         """A handler that will be invoked when the user selects a map pin.
 
-        :param widget: The button that was pressed.
+        :param widget: The MapView that was selected.
         :param pin: The pin that was selected.
         :param kwargs: Ensures compatibility with arguments added in future versions.
         """
-        ...
 
 
 class MapView(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         location: toga.LatLng | tuple[float, float] | None = None,
         zoom: int = 11,
-        pins: list[MapPin] | None = None,
+        pins: Iterable[MapPin] | None = None,
         on_select: toga.widgets.mapview.OnSelectHandler | None = None,
     ):
         """Create a new MapView widget.
@@ -155,7 +155,7 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        self._impl = self.factory.MapView(interface=self)
+        self._impl: Any = self.factory.MapView(interface=self)
 
         self._pins = MapPinSet(self, pins)
 
@@ -179,7 +179,7 @@ def location(self) -> toga.LatLng:
         return self._impl.get_location()
 
     @location.setter
-    def location(self, coordinates: toga.LatLng | tuple[float, float]):
+    def location(self, coordinates: toga.LatLng | tuple[float, float]) -> None:
         self._impl.set_location(toga.LatLng(*coordinates))
 
     @property
@@ -218,7 +218,7 @@ def zoom(self) -> int:
         return round(self._impl.get_zoom())
 
     @zoom.setter
-    def zoom(self, value: int):
+    def zoom(self, value: int) -> None:
         value = int(value)
         if value < 0:
             value = 0
@@ -233,7 +233,7 @@ def pins(self) -> MapPinSet:
         return self._pins
 
     @property
-    def on_select(self) -> toga.widgets.mapview.OnSelectHandler:
+    def on_select(self) -> OnSelectHandler:
         """The handler to invoke when the user selects a pin on a map.
 
         **Note:** This is not currently supported on GTK or Windows.
@@ -241,7 +241,7 @@ def on_select(self) -> toga.widgets.mapview.OnSelectHandler:
         return self._on_select
 
     @on_select.setter
-    def on_select(self, handler: toga.widgets.mapview.OnSelectHandler | None):
+    def on_select(self, handler: toga.widgets.mapview.OnSelectHandler | None) -> None:
         if handler and not getattr(self._impl, "SUPPORTS_ON_SELECT", True):
             self.factory.not_implemented("MapView.on_select")
 
diff --git a/core/src/toga/widgets/multilinetextinput.py b/core/src/toga/widgets/multilinetextinput.py
index d4db065abf..d987725c35 100644
--- a/core/src/toga/widgets/multilinetextinput.py
+++ b/core/src/toga/widgets/multilinetextinput.py
@@ -1,19 +1,31 @@
 from __future__ import annotations
 
+from typing import Any, Protocol
+
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
+
+
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: MultilineTextInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the value is changed.
+
+        :param widget: The MultilineTextInput that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class MultilineTextInput(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         value: str | None = None,
         readonly: bool = False,
         placeholder: str | None = None,
-        on_change: callable | None = None,
+        on_change: toga.widgets.multilinetextinput.OnChangeHandler | None = None,
     ):
         """Create a new multi-line text input widget.
 
@@ -53,7 +65,7 @@ def placeholder(self) -> str:
         return self._impl.get_placeholder()
 
     @placeholder.setter
-    def placeholder(self, value):
+    def placeholder(self, value: object) -> None:
         self._impl.set_placeholder("" if value is None else str(value))
         self.refresh()
 
@@ -68,7 +80,7 @@ def readonly(self) -> bool:
         return self._impl.get_readonly()
 
     @readonly.setter
-    def readonly(self, value):
+    def readonly(self, value: object) -> None:
         self._impl.set_readonly(bool(value))
 
     @property
@@ -81,23 +93,25 @@ def value(self) -> str:
         return self._impl.get_value()
 
     @value.setter
-    def value(self, value):
+    def value(self, value: object) -> None:
         self._impl.set_value("" if value is None else str(value))
         self.refresh()
 
-    def scroll_to_bottom(self):
+    def scroll_to_bottom(self) -> None:
         """Scroll the view to make the bottom of the text field visible."""
         self._impl.scroll_to_bottom()
 
-    def scroll_to_top(self):
+    def scroll_to_top(self) -> None:
         """Scroll the view to make the top of the text field visible."""
         self._impl.scroll_to_top()
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """The handler to invoke when the value of the widget changes."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(
+        self, handler: toga.widgets.multilinetextinput.OnChangeHandler
+    ) -> None:
         self._on_change = wrapped_handler(self, handler)
diff --git a/core/src/toga/widgets/numberinput.py b/core/src/toga/widgets/numberinput.py
index 83690fd36b..8a9907fe67 100644
--- a/core/src/toga/widgets/numberinput.py
+++ b/core/src/toga/widgets/numberinput.py
@@ -1,12 +1,23 @@
 from __future__ import annotations
 
 import re
+import sys
 import warnings
 from decimal import ROUND_HALF_UP, Decimal, InvalidOperation
+from typing import TYPE_CHECKING, Any, Protocol, Union
 
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
+
+if TYPE_CHECKING:
+    if sys.version_info < (3, 10):
+        from typing_extensions import TypeAlias
+    else:
+        from typing import TypeAlias
+
+    NumberInputT: TypeAlias = Union[Decimal, int, float, str]
 
 # Implementation notes
 # ====================
@@ -23,7 +34,7 @@
 NUMERIC_RE = re.compile(r"[^0-9\.-]")
 
 
-def _clean_decimal(value, step=None):
+def _clean_decimal(value: NumberInputT, step: NumberInputT | None = None) -> Decimal:
     # Decimal(3.7) yields "3.700000000...177".
     # However, Decimal(str(3.7)) yields "3.7". If the user provides a float,
     # convert to a string first.
@@ -40,7 +51,7 @@ def _clean_decimal(value, step=None):
     return value
 
 
-def _clean_decimal_str(value):
+def _clean_decimal_str(value: str) -> str:
     """Clean a string value"""
     # Replace any character that isn't a number, `.` or `-`
     value = NUMERIC_RE.sub("", value)
@@ -61,19 +72,28 @@ def _clean_decimal_str(value):
     return value
 
 
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: NumberInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the value is changed.
+
+        :param widget: The NumberInput that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
 class NumberInput(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
-        step: Decimal = 1,
-        min: Decimal | None = None,
-        max: Decimal | None = None,
-        value: Decimal | None = None,
+        id: str | None = None,
+        style: StyleT | None = None,
+        step: NumberInputT = 1,
+        min: NumberInputT | None = None,
+        max: NumberInputT | None = None,
+        value: NumberInputT | None = None,
         readonly: bool = False,
-        on_change: callable | None = None,
-        min_value: Decimal | None = None,  # DEPRECATED
-        max_value: Decimal | None = None,  # DEPRECATED
+        on_change: toga.widgets.numberinput.OnChangeHandler | None = None,
+        min_value: None = None,  # DEPRECATED
+        max_value: None = None,  # DEPRECATED
     ):
         """Create a new number input widget.
 
@@ -88,8 +108,8 @@ def __init__(
             equal to this maximum.
         :param value: The initial value for the widget.
         :param readonly: Can the value of the widget be modified by the user?
-        :param on_change: A handler that will be invoked when the value of the
-            widget changes.
+        :param on_change: A handler that will be invoked when the value of the widget
+            changes.
         :param min_value: **DEPRECATED**; alias of ``min``.
         :param max_value: **DEPRECATED**; alias of ``max``.
         """
@@ -123,8 +143,8 @@ def __init__(
         # The initial setting of min requires calling get_value(),
         # which in turn interrogates min. Prime those values with
         # an empty starting value
-        self._min = None
-        self._max = None
+        self._min: Decimal | None = None
+        self._max: Decimal | None = None
 
         self.on_change = None
         self._impl = self.factory.NumberInput(interface=self)
@@ -148,7 +168,7 @@ def readonly(self) -> bool:
         return self._impl.get_readonly()
 
     @readonly.setter
-    def readonly(self, value):
+    def readonly(self, value: object) -> None:
         self._impl.set_readonly(value)
 
     @property
@@ -160,7 +180,7 @@ def step(self) -> Decimal:
         return self._step
 
     @step.setter
-    def step(self, step):
+    def step(self, step: NumberInputT) -> None:
         try:
             self._step = _clean_decimal(step)
         except (ValueError, TypeError, InvalidOperation):
@@ -168,7 +188,7 @@ def step(self, step):
 
         self._impl.set_step(self._step)
 
-        # Re-assigning the min and max value forces the min/max to be requantized.
+        # Re-assigning the min and max value forces the min/max to be re-quantized.
         self.min = self.min
         self.max = self.max
 
@@ -184,7 +204,7 @@ def min(self) -> Decimal | None:
         return self._min
 
     @min.setter
-    def min(self, new_min):
+    def min(self, new_min: NumberInputT | None) -> None:
         try:
             new_min = _clean_decimal(new_min, self.step)
 
@@ -216,7 +236,7 @@ def max(self) -> Decimal | None:
         return self._max
 
     @max.setter
-    def max(self, new_max):
+    def max(self, new_max: NumberInputT | None) -> None:
         try:
             new_max = _clean_decimal(new_max, self.step)
 
@@ -263,7 +283,7 @@ def value(self) -> Decimal | None:
         return value
 
     @value.setter
-    def value(self, value):
+    def value(self, value: NumberInputT | None) -> None:
         try:
             value = _clean_decimal(value, self.step)
 
@@ -281,12 +301,12 @@ def value(self, value):
         self.refresh()
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """The handler to invoke when the value of the widget changes."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.numberinput.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
     ######################################################################
@@ -303,7 +323,7 @@ def min_value(self) -> Decimal | None:
         return self.min
 
     @min_value.setter
-    def min_value(self, value):
+    def min_value(self, value: NumberInputT | None) -> None:
         warnings.warn(
             "NumberInput.min_value has been renamed NumberInput.min",
             DeprecationWarning,
@@ -320,7 +340,7 @@ def max_value(self) -> Decimal | None:
         return self.max
 
     @max_value.setter
-    def max_value(self, value):
+    def max_value(self, value: NumberInputT | None) -> None:
         warnings.warn(
             "NumberInput.max_value has been renamed NumberInput.max",
             DeprecationWarning,
diff --git a/core/src/toga/widgets/optioncontainer.py b/core/src/toga/widgets/optioncontainer.py
index ebc6621091..8716ebb7c6 100644
--- a/core/src/toga/widgets/optioncontainer.py
+++ b/core/src/toga/widgets/optioncontainer.py
@@ -1,38 +1,37 @@
 from __future__ import annotations
 
 import sys
+from collections.abc import Iterable
 from typing import TYPE_CHECKING, Any, Protocol, overload
 
 import toga
 from toga.handlers import wrapped_handler
 from toga.platform import get_platform_factory
 
-from .base import Widget
+from .base import StyleT, Widget
 
 if TYPE_CHECKING:
     if sys.version_info < (3, 10):
         from typing_extensions import TypeAlias
     else:
         from typing import TypeAlias
+    from toga.icons import IconContentT
 
-    from toga.icons import IconContent
-
-    OptionContainerContent: TypeAlias = (
+    OptionContainerContentT: TypeAlias = (
         tuple[str, Widget]
-        | tuple[str, Widget, IconContent | None]
-        | tuple[str, Widget, IconContent | None, bool]
+        | tuple[str, Widget, IconContentT | None]
+        | tuple[str, Widget, IconContentT | None, bool]
         | toga.OptionItem
     )
 
 
 class OnSelectHandler(Protocol):
-    def __call__(self, widget: OptionContainer, **kwargs: Any) -> None:
+    def __call__(self, widget: OptionContainer, /, **kwargs: Any) -> None:
         """A handler that will be invoked when a new tab is selected in the OptionContainer.
 
         :param widget: The OptionContainer that had a selection change.
         :param kwargs: Ensures compatibility with arguments added in future versions.
         """
-        ...
 
 
 class OptionItem:
@@ -41,14 +40,14 @@ def __init__(
         text: str,
         content: Widget,
         *,
-        icon: IconContent | None = None,
+        icon: IconContentT | None = None,
         enabled: bool = True,
     ):
         """A tab of content in an OptionContainer.
 
         :param text: The text label for the new tab.
         :param content: The content widget to use for the new tab.
-        :param icon: The :any:`icon content <IconContent>` to use to represent the tab.
+        :param icon: The :any:`icon content <IconContentT>` to use to represent the tab.
         :param enabled: Should the new tab be enabled?
         """
         if content is None:
@@ -60,9 +59,9 @@ def __init__(
         # will become the source of truth. Initially prime the attributes with None (so
         # that the attribute exists), then use the setter to enforce validation on the
         # provided values.
-        self._text = None
-        self._icon = None
-        self._enabled = None
+        self._text: str = None
+        self._icon: toga.Icon = None
+        self._enabled: bool = None
 
         self.text = text
         self.icon = icon
@@ -70,8 +69,8 @@ def __init__(
 
         # Prime the attributes for properties that will be set when the OptionItem is
         # set as content.
-        self._interface = None
-        self._index = None
+        self._interface: OptionContainer = None
+        self._index: int = None
 
     @property
     def interface(self) -> OptionContainer:
@@ -90,7 +89,7 @@ def enabled(self) -> bool:
             return self._interface._impl.is_option_enabled(self.index)
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         enable = bool(value)
         if hasattr(self, "_enabled"):
             self._enabled = enable
@@ -112,7 +111,7 @@ def text(self) -> str:
             return self._interface._impl.get_option_text(self.index)
 
     @text.setter
-    def text(self, value):
+    def text(self, value: object) -> None:
         if value is None:
             raise ValueError("Item text cannot be None")
 
@@ -129,7 +128,7 @@ def text(self, value):
     def icon(self) -> toga.Icon:
         """The Icon for the tab of content.
 
-        Can be specified as any valid :any:`icon content <IconContent>`.
+        Can be specified as any valid :any:`icon content <IconContentT>`.
 
         If the platform does not support the display of icons, this property
         will return ``None`` regardless of any value provided.
@@ -140,7 +139,7 @@ def icon(self) -> toga.Icon:
             return self._interface._impl.get_option_icon(self.index)
 
     @icon.setter
-    def icon(self, icon_or_name: IconContent | None):
+    def icon(self, icon_or_name: IconContentT | None) -> None:
         if get_platform_factory().OptionContainer.uses_icons:
             if icon_or_name is None:
                 icon = None
@@ -167,7 +166,7 @@ def content(self) -> Widget:
         """The content widget displayed in this tab of the OptionContainer."""
         return self._content
 
-    def _preserve_option(self):
+    def _preserve_option(self) -> None:
         # Move the ground truth back to the OptionItem instance
         self._text = self.text
         self._icon = self.icon
@@ -177,7 +176,7 @@ def _preserve_option(self):
         self._index = None
         self._interface = None
 
-    def _add_as_option(self, index, interface):
+    def _add_as_option(self, index: int, interface: OptionContainer) -> None:
         text = self._text
         del self._text
 
@@ -198,11 +197,11 @@ def _add_as_option(self, index, interface):
 
 
 class OptionList:
-    def __init__(self, interface):
+    def __init__(self, interface: Any):
         self.interface = interface
-        self._options = []
+        self._options: list[OptionItem] = []
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         items = ", ".join(repr(option.text) for option in self)
         return f"<OptionList {items}>"
 
@@ -210,11 +209,11 @@ def __getitem__(self, index: int | str | OptionItem) -> OptionItem:
         """Obtain a specific tab of content."""
         return self._options[self.index(index)]
 
-    def __delitem__(self, index: int | str | OptionItem):
+    def __delitem__(self, index: int | str | OptionItem) -> None:
         """Same as :any:`remove`."""
         self.remove(index)
 
-    def remove(self, index: int | str | OptionItem):
+    def remove(self, index: int | str | OptionItem) -> None:
         """Remove the specified tab of content.
 
         The currently selected item cannot be deleted.
@@ -244,7 +243,7 @@ def __len__(self) -> int:
         """The number of tabs of content in the OptionContainer."""
         return len(self._options)
 
-    def index(self, value: str | int | OptionItem):
+    def index(self, value: str | int | OptionItem) -> int:
         """Find the index of the tab that matches the given value.
 
         :param value: The value to look for. An integer is returned as-is;
@@ -267,7 +266,7 @@ def index(self, value: str | int | OptionItem):
     def append(
         self,
         text_or_item: OptionItem,
-    ): ...
+    ) -> None: ...
 
     @overload
     def append(
@@ -275,18 +274,18 @@ def append(
         text_or_item: str,
         content: Widget,
         *,
-        icon: IconContent | None = None,
-        enabled: bool = True,
-    ): ...
+        icon: IconContentT | None = None,
+        enabled: bool | None = True,
+    ) -> None: ...
 
     def append(
         self,
-        text_or_item: str,
+        text_or_item: str | OptionItem,
         content: Widget | None = None,
         *,
-        icon: IconContent | None = None,
-        enabled: bool = None,
-    ):
+        icon: IconContentT | None = None,
+        enabled: bool | None = None,
+    ) -> None:
         """Add a new tab of content to the OptionContainer.
 
         The new tab can be specified as an existing :any:`OptionItem` instance, or by
@@ -296,7 +295,7 @@ def append(
 
         :param text_or_item: An :any:`OptionItem`; or, the text label for the new tab.
         :param content: The content widget to use for the new tab.
-        :param icon: The :any:`icon content <IconContent>` to use to represent the tab.
+        :param icon: The :any:`icon content <IconContentT>` to use to represent the tab.
         :param enabled: Should the new tab be enabled? (Default: ``True``)
         """
         self.insert(len(self), text_or_item, content, icon=icon, enabled=enabled)
@@ -306,7 +305,7 @@ def insert(
         self,
         index: int | str | OptionItem,
         text_or_item: OptionItem,
-    ): ...
+    ) -> None: ...
 
     @overload
     def insert(
@@ -315,9 +314,9 @@ def insert(
         text_or_item: str,
         content: Widget,
         *,
-        icon: IconContent | None = None,
-        enabled: bool = True,
-    ): ...
+        icon: IconContentT | None = None,
+        enabled: bool | None = True,
+    ) -> None: ...
 
     def insert(
         self,
@@ -325,9 +324,9 @@ def insert(
         text_or_item: str | OptionItem,
         content: Widget | None = None,
         *,
-        icon: IconContent | None = None,
+        icon: IconContentT | None = None,
         enabled: bool | None = None,
-    ):
+    ) -> None:
         """Insert a new tab of content to the OptionContainer at the specified index.
 
         The new tab can be specified as an existing :any:`OptionItem` instance, or by
@@ -338,7 +337,7 @@ def insert(
         :param index: The index where the new tab should be inserted.
         :param text_or_item: An :any:`OptionItem`; or, the text label for the new tab.
         :param content: The content widget to use for the new tab.
-        :param icon: The :any:`icon content <IconContent>` to use to represent the tab.
+        :param icon: The :any:`icon content <IconContentT>` to use to represent the tab.
         :param enabled: Should the new tab be enabled? (Default: ``True``)
         """
         if isinstance(text_or_item, OptionItem):
@@ -379,9 +378,9 @@ def insert(
 class OptionContainer(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
-        content: list[OptionContainerContent] | None = None,
+        id: str | None = None,
+        style: StyleT | None = None,
+        content: Iterable[OptionContainerContentT] | None = None,
         on_select: toga.widgets.optioncontainer.OnSelectHandler | None = None,
     ):
         """Create a new OptionContainer.
@@ -390,7 +389,7 @@ def __init__(
         :param style: A style object. If no style is provided, a default style will be
             applied to the widget.
         :param content: The initial :any:`OptionContainer content
-            <OptionContainerContent>` to display in the OptionContainer.
+            <OptionContainerContentT>` to display in the OptionContainer.
         :param on_select: Initial :any:`on_select` handler.
         """
         super().__init__(id=id, style=style)
@@ -399,7 +398,7 @@ def __init__(
 
         self._impl = self.factory.OptionContainer(interface=self)
 
-        if content:
+        if content is not None:
             for item in content:
                 if isinstance(item, OptionItem):
                     self.content.append(item)
@@ -434,12 +433,11 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        """No-op; OptionContainer cannot accept input focus"""
-        pass
+    def focus(self) -> None:
+        """No-op; OptionContainer cannot accept input focus."""
 
     @property
     def content(self) -> OptionList:
@@ -459,7 +457,7 @@ def current_tab(self) -> OptionItem | None:
         return self._content[index]
 
     @current_tab.setter
-    def current_tab(self, value):
+    def current_tab(self, value: OptionItem | str | int) -> None:
         index = self._content.index(value)
         if not self._impl.is_option_enabled(index):
             raise ValueError("A disabled tab cannot be made the current tab.")
@@ -467,7 +465,7 @@ def current_tab(self, value):
         self._impl.set_current_tab_index(index)
 
     @Widget.app.setter
-    def app(self, app):
+    def app(self, app) -> None:
         # Invoke the superclass property setter
         Widget.app.fset(self, app)
 
@@ -476,7 +474,7 @@ def app(self, app):
             item._content.app = app
 
     @Widget.window.setter
-    def window(self, window):
+    def window(self, window) -> None:
         # Invoke the superclass property setter
         Widget.window.fset(self, window)
 
@@ -485,10 +483,10 @@ def window(self, window):
             item._content.window = window
 
     @property
-    def on_select(self) -> toga.widgets.optioncontainer.OnSelectHandler:
+    def on_select(self) -> OnSelectHandler:
         """The callback to invoke when a new tab of content is selected."""
         return self._on_select
 
     @on_select.setter
-    def on_select(self, handler):
+    def on_select(self, handler: toga.widgets.optioncontainer.OnSelectHandler) -> None:
         self._on_select = wrapped_handler(self, handler)
diff --git a/core/src/toga/widgets/passwordinput.py b/core/src/toga/widgets/passwordinput.py
index 8044527841..48548b4935 100644
--- a/core/src/toga/widgets/passwordinput.py
+++ b/core/src/toga/widgets/passwordinput.py
@@ -6,5 +6,5 @@
 class PasswordInput(TextInput):
     """Create a new password input widget."""
 
-    def _create(self):
+    def _create(self) -> None:
         self._impl = self.factory.PasswordInput(interface=self)
diff --git a/core/src/toga/widgets/progressbar.py b/core/src/toga/widgets/progressbar.py
index 31bb1e80e5..49f2e67575 100644
--- a/core/src/toga/widgets/progressbar.py
+++ b/core/src/toga/widgets/progressbar.py
@@ -1,6 +1,8 @@
 from __future__ import annotations
 
-from .base import Widget
+from typing import Literal, SupportsFloat
+
+from .base import StyleT, Widget
 
 
 class ProgressBar(Widget):
@@ -8,10 +10,10 @@ class ProgressBar(Widget):
 
     def __init__(
         self,
-        id=None,
-        style=None,
-        max: float = 1.0,
-        value: float = 0.0,
+        id: str | None = None,
+        style: StyleT | None = None,
+        max: str | SupportsFloat = 1.0,
+        value: str | SupportsFloat = 0.0,
         running: bool = False,
     ):
         """Create a new Progress Bar widget.
@@ -39,7 +41,7 @@ def __init__(
             self.start()
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
 
         ProgressBar widgets cannot be disabled; this property will always return True;
@@ -48,7 +50,7 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
     @property
@@ -69,7 +71,7 @@ def is_determinate(self) -> bool:
         """
         return self.max is not None
 
-    def start(self):
+    def start(self) -> None:
         """Start the progress bar.
 
         If the progress bar is already started, this is a no-op.
@@ -77,7 +79,7 @@ def start(self):
         if not self.is_running:
             self._impl.start()
 
-    def stop(self):
+    def stop(self) -> None:
         """Stop the progress bar.
 
         If the progress bar is already stopped, this is a no-op.
@@ -98,7 +100,7 @@ def value(self) -> float:
         return self._impl.get_value()
 
     @value.setter
-    def value(self, value):
+    def value(self, value: str | SupportsFloat) -> None:
         if self.max is not None:
             value = max(0.0, min(self.max, float(value)))
             self._impl.set_value(value)
@@ -112,7 +114,7 @@ def max(self) -> float | None:
         return self._impl.get_max()
 
     @max.setter
-    def max(self, value):
+    def max(self, value: str | SupportsFloat | None) -> None:
         if value is None:
             self._impl.set_max(None)
         elif float(value) > 0.0:
diff --git a/core/src/toga/widgets/scrollcontainer.py b/core/src/toga/widgets/scrollcontainer.py
index 4d1a64d36e..030cbc1bf9 100644
--- a/core/src/toga/widgets/scrollcontainer.py
+++ b/core/src/toga/widgets/scrollcontainer.py
@@ -1,24 +1,33 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any, Literal, Protocol, SupportsInt
 
 from toga.handlers import wrapped_handler
 from toga.types import Position
 
-from .base import Widget
+from .base import StyleT, Widget
 
 if TYPE_CHECKING:
     from toga.types import PositionT
 
 
+class OnScrollHandler(Protocol):
+    def __call__(self, widget: ScrollContainer, /, **kwargs: Any) -> object:
+        """A handler to invoke when the container is scrolled.
+
+        :param widget: The ScrollContainer that was scrolled.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
 class ScrollContainer(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         horizontal: bool = True,
         vertical: bool = True,
-        on_scroll: callable | None = None,
+        on_scroll: OnScrollHandler | None = None,
         content: Widget | None = None,
     ):
         """Create a new Scroll Container.
@@ -33,8 +42,9 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        self._content = None
+        self._content: Widget | None = None
         self.on_scroll = None
+
         # Create a platform specific implementation of a Scroll Container
         self._impl = self.factory.ScrollContainer(interface=self)
 
@@ -45,7 +55,7 @@ def __init__(
         self.on_scroll = on_scroll
 
     @Widget.app.setter
-    def app(self, app):
+    def app(self, app) -> None:
         # Invoke the superclass property setter
         Widget.app.fset(self, app)
 
@@ -54,7 +64,7 @@ def app(self, app):
             self._content.app = app
 
     @Widget.window.setter
-    def window(self, window):
+    def window(self, window) -> None:
         # Invoke the superclass property setter
         Widget.window.fset(self, window)
 
@@ -63,7 +73,7 @@ def window(self, window):
             self._content.window = window
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
 
         ScrollContainer widgets cannot be disabled; this property will always return
@@ -72,20 +82,20 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; ScrollContainer cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; ScrollContainer cannot accept input focus."""
         pass
 
     @property
-    def content(self) -> Widget:
+    def content(self) -> Widget | None:
         """The root content widget displayed inside the scroll container."""
         return self._content
 
     @content.setter
-    def content(self, widget):
+    def content(self, widget: Widget | None) -> None:
         if self._content:
             # Clear the window before the app so that registry entries can be cleared
             self._content.window = None
@@ -108,7 +118,7 @@ def vertical(self) -> bool:
         return self._impl.get_vertical()
 
     @vertical.setter
-    def vertical(self, value):
+    def vertical(self, value: object) -> None:
         self._impl.set_vertical(bool(value))
         if self._content:
             self._content.refresh()
@@ -119,18 +129,18 @@ def horizontal(self) -> bool:
         return self._impl.get_horizontal()
 
     @horizontal.setter
-    def horizontal(self, value):
+    def horizontal(self, value: object) -> None:
         self._impl.set_horizontal(bool(value))
         if self._content:
             self._content.refresh()
 
     @property
-    def on_scroll(self) -> callable:
+    def on_scroll(self) -> OnScrollHandler:
         """Handler to invoke when the user moves a scroll bar."""
         return self._on_scroll
 
     @on_scroll.setter
-    def on_scroll(self, on_scroll):
+    def on_scroll(self, on_scroll: OnScrollHandler) -> None:
         self._on_scroll = wrapped_handler(self, on_scroll)
 
     @property
@@ -155,7 +165,7 @@ def horizontal_position(self) -> int:
         return self._impl.get_horizontal_position()
 
     @horizontal_position.setter
-    def horizontal_position(self, horizontal_position):
+    def horizontal_position(self, horizontal_position: SupportsInt) -> None:
         if not self.horizontal:
             raise ValueError(
                 "Cannot set horizontal position when horizontal scrolling is not enabled."
@@ -185,7 +195,7 @@ def vertical_position(self) -> int:
         return self._impl.get_vertical_position()
 
     @vertical_position.setter
-    def vertical_position(self, vertical_position):
+    def vertical_position(self, vertical_position: SupportsInt) -> None:
         if not self.vertical:
             raise ValueError(
                 "Cannot set vertical position when vertical scrolling is not enabled."
diff --git a/core/src/toga/widgets/selection.py b/core/src/toga/widgets/selection.py
index 83a9ae3bf1..9b9fb5d565 100644
--- a/core/src/toga/widgets/selection.py
+++ b/core/src/toga/widgets/selection.py
@@ -1,24 +1,38 @@
 from __future__ import annotations
 
 import warnings
+from collections.abc import Iterable
+from typing import Any, Protocol, TypeVar
 
+import toga
 from toga.handlers import wrapped_handler
 from toga.sources import ListSource, Source
 
-from .base import Widget
+from .base import StyleT, Widget
+
+SourceT = TypeVar("SourceT", bound=Source)
+
+
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: Selection, /, **kwargs: Any) -> object:
+        """A handler to invoke when the value is changed.
+
+        :param widget: The Selection that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class Selection(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
-        items: list | ListSource | None = None,
+        id: str | None = None,
+        style: StyleT | None = None,
+        items: SourceT | Iterable | None = None,
         accessor: str | None = None,
-        value: None = None,
-        on_change: callable | None = None,
-        enabled=True,
-        on_select: callable | None = None,  # DEPRECATED
+        value: object | None = None,
+        on_change: toga.widgets.selection.OnChangeHandler | None = None,
+        enabled: bool = True,
+        on_select: None = None,  # DEPRECATED
     ):
         """Create a new Selection widget.
 
@@ -52,6 +66,8 @@ def __init__(
         # End backwards compatibility.
         ######################################################################
 
+        self._items: SourceT | ListSource
+
         self.on_change = None  # needed for _impl initialization
         self._impl = self.factory.Selection(interface=self)
 
@@ -64,7 +80,7 @@ def __init__(
         self.enabled = enabled
 
     @property
-    def items(self) -> ListSource:
+    def items(self) -> SourceT | ListSource:
         """The items to display in the selection.
 
         When setting this property:
@@ -81,7 +97,7 @@ def items(self) -> ListSource:
         return self._items
 
     @items.setter
-    def items(self, items):
+    def items(self, items: SourceT | Iterable | None) -> None:
         if self._accessor is None:
             accessors = ["value"]
         else:
@@ -113,7 +129,7 @@ def items(self, items):
 
         self.refresh()
 
-    def _title_for_item(self, item):
+    def _title_for_item(self, item: Any) -> str:
         """Internal utility method; return the display title for an item"""
         if self._accessor:
             title = getattr(item, self._accessor)
@@ -123,7 +139,7 @@ def _title_for_item(self, item):
         return str(title).split("\n")[0]
 
     @property
-    def value(self):
+    def value(self) -> object | None:
         """The currently selected item.
 
         Returns None if there are no items in the selection.
@@ -151,7 +167,7 @@ def value(self):
         return item
 
     @value.setter
-    def value(self, value):
+    def value(self, value: object) -> None:
         try:
             if self._accessor is None:
                 item = self._items.find(dict(value=value))
@@ -164,13 +180,13 @@ def value(self, value):
             raise ValueError(f"{value!r} is not a current item in the selection")
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """Handler to invoke when the value of the selection is changed, either by the user
         or programmatically."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.selection.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
     ######################################################################
@@ -178,7 +194,7 @@ def on_change(self, handler):
     ######################################################################
 
     @property
-    def on_select(self) -> callable:
+    def on_select(self) -> OnChangeHandler:
         """**DEPRECATED**: Use ``on_change``"""
         warnings.warn(
             "Selection.on_select has been renamed Selection.on_change.",
@@ -187,7 +203,7 @@ def on_select(self) -> callable:
         return self.on_change
 
     @on_select.setter
-    def on_select(self, handler):
+    def on_select(self, handler: toga.widgets.selection.OnChangeHandler) -> None:
         warnings.warn(
             "Selection.on_select has been renamed Selection.on_change.",
             DeprecationWarning,
diff --git a/core/src/toga/widgets/slider.py b/core/src/toga/widgets/slider.py
index 98e559483f..efddce2312 100644
--- a/core/src/toga/widgets/slider.py
+++ b/core/src/toga/widgets/slider.py
@@ -3,26 +3,55 @@
 import warnings
 from abc import ABC, abstractmethod
 from contextlib import contextmanager
+from typing import Any, Protocol, SupportsFloat
 
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
+
+
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: Slider, /, **kwargs: Any) -> object:
+        """A handler to invoke when the value is changed.
+
+        :param widget: The Slider that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnPressHandler(Protocol):
+    def __call__(self, widget: Slider, /, **kwargs: Any) -> object:
+        """A handler to invoke when the slider is pressed.
+
+        :param widget: The Slider that was pressed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnReleaseHandler(Protocol):
+    def __call__(self, widget: Slider, /, **kwargs: Any) -> object:
+        """A handler to invoke when the slider is pressed.
+
+        :param widget: The Slider that was released.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class Slider(Widget):
     def __init__(
         self,
         id: str | None = None,
-        style=None,
+        style: StyleT | None = None,
         value: float | None = None,
-        min: float = None,  # Default to 0.0 when range is removed
-        max: float = None,  # Default to 1.0 when range is removed
+        min: float | None = None,  # Default to 0.0 when range is removed
+        max: float | None = None,  # Default to 1.0 when range is removed
         tick_count: int | None = None,
-        on_change: callable | None = None,
-        on_press: callable | None = None,
-        on_release: callable | None = None,
+        on_change: toga.widgets.slider.OnChangeHandler | None = None,
+        on_press: toga.widgets.slider.OnPressHandler | None = None,
+        on_release: OnReleaseHandler | None = None,
         enabled: bool = True,
-        range: tuple[float, float] | None = None,  # DEPRECATED
+        range: None = None,  # DEPRECATED
     ):
         """Create a new Slider widget.
 
@@ -90,7 +119,7 @@ def __init__(
     # Backends are inconsistent about when they produce events for programmatic changes,
     # so we deal with those in the interface layer.
     @contextmanager
-    def _programmatic_change(self):
+    def _programmatic_change(self) -> float:
         old_value = self.value
         on_change = self._on_change
         self.on_change = None
@@ -111,7 +140,7 @@ def value(self) -> float:
         return self._impl.get_value()
 
     @value.setter
-    def value(self, value):
+    def value(self, value: float) -> None:
         if value < self.min:
             value = self.min
         elif value > self.max:
@@ -119,10 +148,10 @@ def value(self, value):
         with self._programmatic_change():
             self._set_value(value)
 
-    def _set_value(self, value):
+    def _set_value(self, value: SupportsFloat) -> None:
         self._impl.set_value(self._round_value(float(value)))
 
-    def _round_value(self, value):
+    def _round_value(self, value: float) -> float:
         step = self.tick_step
         if step is not None:
             # Round to the nearest tick.
@@ -139,7 +168,7 @@ def min(self) -> float:
         return self._impl.get_min()
 
     @min.setter
-    def min(self, value):
+    def min(self, value: SupportsFloat) -> None:
         with self._programmatic_change() as old_value:
             # Some backends will clip the current value within the range automatically,
             # but do it ourselves to be certain. In discrete mode, setting self.value also
@@ -163,7 +192,7 @@ def max(self) -> float:
         return self._impl.get_max()
 
     @max.setter
-    def max(self, value):
+    def max(self, value: SupportsFloat) -> None:
         with self._programmatic_change() as old_value:
             # Some backends will clip the current value within the range automatically,
             # but do it ourselves to be certain. In discrete mode, setting self.value also
@@ -199,7 +228,7 @@ def tick_count(self) -> int | None:
         return self._impl.get_tick_count()
 
     @tick_count.setter
-    def tick_count(self, tick_count):
+    def tick_count(self, tick_count: float | None) -> None:
         if (tick_count is not None) and (tick_count < 2):
             raise ValueError("tick count must be at least 2")
         with self._programmatic_change() as old_value:
@@ -226,7 +255,7 @@ def tick_step(self) -> float | None:
         return (self.max - self.min) / (self.tick_count - 1)
 
     @property
-    def tick_value(self) -> int | None:
+    def tick_value(self) -> float | None:
         """Value of the slider, measured in ticks.
 
         * If the slider is continuous, this property returns ``None``.
@@ -235,25 +264,25 @@ def tick_value(self) -> int | None:
 
         :raises ValueError: If set to anything inconsistent with the rules above.
         """
-        if self.tick_count is not None:
+        if self.tick_count is not None and self.tick_step is not None:
             return round((self.value - self.min) / self.tick_step) + 1
         else:
             return None
 
     @tick_value.setter
-    def tick_value(self, tick_value):
+    def tick_value(self, tick_value: int | None) -> None:
         if self.tick_count is None:
             if tick_value is not None:
                 raise ValueError("cannot set tick value when tick count is None")
         else:
-            if tick_value is None:
+            if tick_value is None or self.tick_step is None:
                 raise ValueError(
                     "cannot set tick value to None when tick count is not None"
                 )
             self.value = self.min + (tick_value - 1) * self.tick_step
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """Handler to invoke when the value of the slider is changed, either by the user
         or programmatically.
 
@@ -262,25 +291,25 @@ def on_change(self) -> callable:
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.slider.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
     @property
-    def on_press(self) -> callable:
+    def on_press(self) -> OnPressHandler:
         """Handler to invoke when the user presses the slider before changing it."""
         return self._on_press
 
     @on_press.setter
-    def on_press(self, handler):
+    def on_press(self, handler: toga.widgets.slider.OnPressHandler) -> None:
         self._on_press = wrapped_handler(self, handler)
 
     @property
-    def on_release(self) -> callable:
+    def on_release(self) -> OnReleaseHandler:
         """Handler to invoke when the user releases the slider after changing it."""
         return self._on_release
 
     @on_release.setter
-    def on_release(self, handler):
+    def on_release(self, handler: OnReleaseHandler) -> None:
         self._on_release = wrapped_handler(self, handler)
 
     ######################################################################
@@ -303,10 +332,10 @@ def range(self) -> tuple[float, float]:
             "Slider.range has been deprecated in favor of Slider.min and Slider.max",
             DeprecationWarning,
         )
-        return (self.min, self.max)
+        return self.min, self.max
 
     @range.setter
-    def range(self, range):
+    def range(self, range: tuple[float, float]) -> None:
         warnings.warn(
             "Slider.range has been deprecated in favor of Slider.min and Slider.max",
             DeprecationWarning,
@@ -317,29 +346,31 @@ def range(self, range):
 
 
 class SliderImpl(ABC):
+    interface: Any
+
     @abstractmethod
-    def get_value(self): ...
+    def get_value(self) -> float: ...
 
     @abstractmethod
-    def set_value(self, value): ...
+    def set_value(self, value: float) -> None: ...
 
     @abstractmethod
-    def get_min(self): ...
+    def get_min(self) -> float: ...
 
     @abstractmethod
-    def set_min(self, value): ...
+    def set_min(self, value: float) -> None: ...
 
     @abstractmethod
-    def get_max(self): ...
+    def get_max(self) -> float: ...
 
     @abstractmethod
-    def set_max(self, value): ...
+    def set_max(self, value: float) -> None: ...
 
     @abstractmethod
-    def get_tick_count(self): ...
+    def get_tick_count(self) -> int | None: ...
 
     @abstractmethod
-    def set_tick_count(self, tick_count): ...
+    def set_tick_count(self, tick_count: int | None) -> None: ...
 
 
 class IntSliderImpl(SliderImpl):
@@ -348,41 +379,41 @@ class IntSliderImpl(SliderImpl):
     # Number of steps to use to approximate a continuous slider.
     CONTINUOUS_MAX = 10000
 
-    def __init__(self):
+    def __init__(self) -> None:
         super().__init__()
 
         # Dummy values used during initialization.
-        self.value = 0
-        self.min = 0
-        self.max = 1
+        self.value: int | float = 0
+        self.min: int | float = 0
+        self.max: int | float = 1
         self.discrete = False
 
-    def get_value(self):
+    def get_value(self) -> float:
         return self.value
 
-    def set_value(self, value):
+    def set_value(self, value: float) -> None:
         span = self.max - self.min
         self.set_int_value(
             0 if span == 0 else round((value - self.min) / span * self.get_int_max())
         )
         self.value = value  # Cache the original value so we can round-trip it.
 
-    def get_min(self):
+    def get_min(self) -> float:
         return self.min
 
-    def set_min(self, value):
+    def set_min(self, value: float) -> None:
         self.min = value
 
-    def get_max(self):
+    def get_max(self) -> float:
         return self.max
 
-    def set_max(self, value):
+    def set_max(self, value: float) -> None:
         self.max = value
 
-    def get_tick_count(self):
+    def get_tick_count(self) -> int | None:
         return (self.get_int_max() + 1) if self.discrete else None
 
-    def set_tick_count(self, tick_count):
+    def set_tick_count(self, tick_count: int | None) -> None:
         if tick_count is None:
             self.discrete = False
             self.set_int_max(self.CONTINUOUS_MAX)
@@ -393,22 +424,22 @@ def set_tick_count(self, tick_count):
 
     # Instead of calling the event handler directly, implementations should call this
     # method.
-    def on_change(self):
+    def on_change(self) -> None:
         span = self.max - self.min
         self.value = self.min + (self.get_int_value() / self.get_int_max() * span)
         self.interface.on_change()
 
     @abstractmethod
-    def get_int_value(self): ...
+    def get_int_value(self) -> int: ...
 
     @abstractmethod
-    def set_int_value(self, value): ...
+    def set_int_value(self, value: int) -> None: ...
 
     @abstractmethod
-    def get_int_max(self): ...
+    def get_int_max(self) -> int: ...
 
     @abstractmethod
-    def set_int_max(self, max): ...
+    def set_int_max(self, max: int) -> None: ...
 
     @abstractmethod
-    def set_ticks_visible(self, visible): ...
+    def set_ticks_visible(self, visible: bool) -> None: ...
diff --git a/core/src/toga/widgets/splitcontainer.py b/core/src/toga/widgets/splitcontainer.py
index 79b29e3789..04589bc737 100644
--- a/core/src/toga/widgets/splitcontainer.py
+++ b/core/src/toga/widgets/splitcontainer.py
@@ -1,8 +1,21 @@
 from __future__ import annotations
 
+import sys
+from typing import TYPE_CHECKING
+
+from toga.app import App
 from toga.constants import Direction
+from toga.window import Window
+
+from .base import StyleT, Widget
 
-from .base import Widget
+if TYPE_CHECKING:
+    if sys.version_info < (3, 10):
+        from typing_extensions import TypeAlias
+    else:
+        from typing import TypeAlias
+
+    SplitContainerContentT: TypeAlias = Widget | tuple[Widget, float] | None
 
 
 class SplitContainer(Widget):
@@ -11,10 +24,10 @@ class SplitContainer(Widget):
 
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         direction: Direction = Direction.VERTICAL,
-        content: tuple[Widget | None | tuple, Widget | None | tuple] = (None, None),
+        content: tuple[SplitContainerContentT, SplitContainerContentT] = (None, None),
     ):
         """Create a new SplitContainer.
 
@@ -25,11 +38,14 @@ def __init__(
             :attr:`~toga.constants.Direction.HORIZONTAL` or
             :attr:`~toga.constants.Direction.VERTICAL`; defaults to
             :attr:`~toga.constants.Direction.VERTICAL`
-        :param content: Initial :any:`content` of the container. Defaults to both panels
-            being empty.
+        :param content: Initial :any:`SplitContainer content <SplitContainerContentT>`
+            of the container. Defaults to both panels being empty.
         """
         super().__init__(id=id, style=style)
-        self._content = (None, None)
+        self._content: tuple[SplitContainerContentT, SplitContainerContentT] = (
+            None,
+            None,
+        )
 
         # Create a platform specific implementation of a SplitContainer
         self._impl = self.factory.SplitContainer(interface=self)
@@ -47,17 +63,15 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; SplitContainer cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; SplitContainer cannot accept input focus."""
         pass
 
-    # The inner tuple's full type is tuple[Widget | None, float], but that would make
-    # the documentation unreadable.
     @property
-    def content(self) -> tuple[Widget | None | tuple, Widget | None | tuple]:
+    def content(self) -> tuple[SplitContainerContentT, SplitContainerContentT]:
         """The widgets displayed in the SplitContainer.
 
         This property accepts a sequence of exactly 2 elements, each of which can be
@@ -75,7 +89,9 @@ def content(self) -> tuple[Widget | None | tuple, Widget | None | tuple]:
         return self._content
 
     @content.setter
-    def content(self, content):
+    def content(
+        self, content: tuple[SplitContainerContentT, SplitContainerContentT]
+    ) -> None:
         try:
             if len(content) != 2:
                 raise TypeError()
@@ -119,7 +135,7 @@ def content(self, content):
         self.refresh()
 
     @Widget.app.setter
-    def app(self, app):
+    def app(self, app: App | None) -> None:
         # Invoke the superclass property setter
         Widget.app.fset(self, app)
 
@@ -129,7 +145,7 @@ def app(self, app):
                 content.app = app
 
     @Widget.window.setter
-    def window(self, window):
+    def window(self, window: Window | None) -> None:
         # Invoke the superclass property setter
         Widget.window.fset(self, window)
 
@@ -140,10 +156,10 @@ def window(self, window):
 
     @property
     def direction(self) -> Direction:
-        """The direction of the split"""
+        """The direction of the split."""
         return self._impl.get_direction()
 
     @direction.setter
-    def direction(self, value):
+    def direction(self, value: object) -> None:
         self._impl.set_direction(value)
         self.refresh()
diff --git a/core/src/toga/widgets/switch.py b/core/src/toga/widgets/switch.py
index b5aa934603..17c0dfb354 100644
--- a/core/src/toga/widgets/switch.py
+++ b/core/src/toga/widgets/switch.py
@@ -1,17 +1,29 @@
 from __future__ import annotations
 
+from typing import Any, Protocol
+
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
+
+
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: Switch, /, **kwargs: Any) -> object:
+        """A handler to invoke when the value is changed.
+
+        :param widget: The Switch that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class Switch(Widget):
     def __init__(
         self,
-        text,
-        id=None,
-        style=None,
-        on_change: callable | None = None,
+        text: str,
+        id: str | None = None,
+        style: StyleT | None = None,
+        on_change: toga.widgets.switch.OnChangeHandler | None = None,
         value: bool = False,
         enabled: bool = True,
     ):
@@ -56,7 +68,7 @@ def text(self) -> str:
         return self._impl.get_text()
 
     @text.setter
-    def text(self, value):
+    def text(self, value: object) -> None:
         if value is None or value == "\u200B":
             value = ""
         else:
@@ -68,12 +80,12 @@ def text(self, value):
         self.refresh()
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """The handler to invoke when the value of the switch changes."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.switch.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
     @property
@@ -85,9 +97,9 @@ def value(self) -> bool:
         return self._impl.get_value()
 
     @value.setter
-    def value(self, value):
+    def value(self, value: object) -> None:
         self._impl.set_value(bool(value))
 
-    def toggle(self):
+    def toggle(self) -> None:
         """Reverse the current value of the switch."""
         self.value = not self.value
diff --git a/core/src/toga/widgets/table.py b/core/src/toga/widgets/table.py
index c63de105c6..d0e991e0fb 100644
--- a/core/src/toga/widgets/table.py
+++ b/core/src/toga/widgets/table.py
@@ -1,28 +1,51 @@
 from __future__ import annotations
 
 import warnings
-from typing import Any
+from collections.abc import Iterable
+from typing import Any, Literal, Protocol, TypeVar
 
+import toga
 from toga.handlers import wrapped_handler
 from toga.sources import ListSource, Row, Source
 from toga.sources.accessors import build_accessors, to_accessor
 
-from .base import Widget
+from .base import StyleT, Widget
+
+SourceT = TypeVar("SourceT", bound=Source)
+
+
+class OnSelectHandler(Protocol):
+    def __call__(self, widget: Table, /, **kwargs: Any) -> object:
+        """A handler to invoke when the table is selected.
+
+        :param widget: The Table that was selected.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnActivateHandler(Protocol):
+    def __call__(self, widget: Table, /, row: Any, **kwargs: Any) -> object:
+        """A handler to invoke when the table is activated.
+
+        :param widget: The Table that was activated.
+        :param row: The Table Row that was activated.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class Table(Widget):
     def __init__(
         self,
-        headings: list[str] | None = None,
-        id=None,
-        style=None,
-        data: Any = None,
-        accessors: list[str] | None = None,
+        headings: Iterable[str] | None = None,
+        id: str | None = None,
+        style: StyleT | None = None,
+        data: SourceT | Iterable | None = None,
+        accessors: Iterable[str] | None = None,
         multiple_select: bool = False,
-        on_select: callable | None = None,
-        on_activate: callable | None = None,
+        on_select: toga.widgets.table.OnSelectHandler | None = None,
+        on_activate: toga.widgets.table.OnActivateHandler | None = None,
         missing_value: str = "",
-        on_double_click=None,  # DEPRECATED
+        on_double_click: None = None,  # DEPRECATED
     ):
         """Create a new Table widget.
 
@@ -73,12 +96,16 @@ def __init__(
         # End backwards compatibility.
         ######################################################################
 
+        self._headings: list[str] | None
+        self._accessors: list[str]
+        self._data: SourceT | ListSource
+
         if headings is not None:
             self._headings = [heading.split("\n")[0] for heading in headings]
             self._accessors = build_accessors(self._headings, accessors)
         elif accessors is not None:
             self._headings = None
-            self._accessors = accessors
+            self._accessors = list(accessors)
         else:
             raise ValueError(
                 "Cannot create a table without either headings or accessors"
@@ -99,7 +126,7 @@ def __init__(
         self.on_activate = on_activate
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
         Table widgets cannot be disabled; this property will always return True; any
         attempt to modify it will be ignored.
@@ -107,15 +134,15 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; Table cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; Table cannot accept input focus."""
         pass
 
     @property
-    def data(self) -> ListSource:
+    def data(self) -> SourceT | ListSource:
         """The data to display in the table.
 
         When setting this property:
@@ -131,7 +158,7 @@ def data(self) -> ListSource:
         return self._data
 
     @data.setter
-    def data(self, data: Any):
+    def data(self, data: SourceT | Iterable | None) -> None:
         if data is None:
             self._data = ListSource(accessors=self._accessors, data=[])
         elif isinstance(data, Source):
@@ -166,11 +193,11 @@ def selection(self) -> list[Row] | Row | None:
         else:
             return self.data[selection]
 
-    def scroll_to_top(self):
+    def scroll_to_top(self) -> None:
         """Scroll the view so that the top of the list (first row) is visible."""
         self.scroll_to_row(0)
 
-    def scroll_to_row(self, row: int):
+    def scroll_to_row(self, row: int) -> None:
         """Scroll the view so that the specified row index is visible.
 
         :param row: The index of the row to make visible. Negative values refer to the
@@ -182,34 +209,34 @@ def scroll_to_row(self, row: int):
             else:
                 self._impl.scroll_to_row(max(len(self.data) + row, 0))
 
-    def scroll_to_bottom(self):
+    def scroll_to_bottom(self) -> None:
         """Scroll the view so that the bottom of the list (last row) is visible."""
         self.scroll_to_row(-1)
 
     @property
-    def on_select(self) -> callable:
+    def on_select(self) -> OnSelectHandler:
         """The callback function that is invoked when a row of the table is selected."""
         return self._on_select
 
     @on_select.setter
-    def on_select(self, handler: callable):
+    def on_select(self, handler: toga.widgets.table.OnSelectHandler) -> None:
         self._on_select = wrapped_handler(self, handler)
 
     @property
-    def on_activate(self) -> callable:
+    def on_activate(self) -> OnActivateHandler:
         """The callback function that is invoked when a row of the table is activated,
         usually with a double click or similar action."""
         return self._on_activate
 
     @on_activate.setter
-    def on_activate(self, handler):
+    def on_activate(self, handler: toga.widgets.table.OnActivateHandler) -> None:
         self._on_activate = wrapped_handler(self, handler)
 
-    def add_column(self, heading: str, accessor: str | None = None):
+    def add_column(self, heading: str, accessor: str | None = None) -> None:
         """**DEPRECATED**: use :meth:`~toga.Table.append_column`"""
         self.insert_column(len(self._accessors), heading, accessor=accessor)
 
-    def append_column(self, heading: str, accessor: str | None = None):
+    def append_column(self, heading: str, accessor: str | None = None) -> None:
         """Append a column to the end of the table.
 
         :param heading: The heading for the new column.
@@ -222,9 +249,9 @@ def append_column(self, heading: str, accessor: str | None = None):
     def insert_column(
         self,
         index: int | str,
-        heading: str | None,
+        heading: str,
         accessor: str | None = None,
-    ):
+    ) -> None:
         """Insert an additional column into the table.
 
         :param index: The index at which to insert the column, or the accessor of the
@@ -257,7 +284,7 @@ def insert_column(
 
         self._impl.insert_column(index, heading, accessor)
 
-    def remove_column(self, column: int | str):
+    def remove_column(self, column: int | str) -> None:
         """Remove a table column.
 
         :param column: The index of the column to remove, or the accessor of the column
@@ -302,7 +329,7 @@ def missing_value(self) -> str:
     ######################################################################
 
     @property
-    def on_double_click(self):
+    def on_double_click(self) -> OnActivateHandler:
         """**DEPRECATED**: Use ``on_activate``"""
         warnings.warn(
             "Table.on_double_click has been renamed Table.on_activate.",
@@ -311,7 +338,7 @@ def on_double_click(self):
         return self.on_activate
 
     @on_double_click.setter
-    def on_double_click(self, handler):
+    def on_double_click(self, handler: toga.widgets.table.OnActivateHandler) -> None:
         warnings.warn(
             "Table.on_double_click has been renamed Table.on_activate.",
             DeprecationWarning,
diff --git a/core/src/toga/widgets/textinput.py b/core/src/toga/widgets/textinput.py
index 5c207e7edc..cea1d1e44a 100644
--- a/core/src/toga/widgets/textinput.py
+++ b/core/src/toga/widgets/textinput.py
@@ -1,8 +1,48 @@
 from __future__ import annotations
 
+from collections.abc import Callable, Iterable
+from typing import Any, Protocol
+
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
+
+
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: TextInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the text input is changed.
+
+        :param widget: The TextInput that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnConfirmHandler(Protocol):
+    def __call__(self, widget: TextInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the text input is confirmed.
+
+        :param widget: The TextInput that was confirmed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnGainFocusHandler(Protocol):
+    def __call__(self, widget: TextInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the text input gains focus.
+
+        :param widget: The TextInput that gained focus.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnLoseFocusHandler(Protocol):
+    def __call__(self, widget: TextInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the text input loses focus.
+
+        :param widget: The TextInput that lost focus.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class TextInput(Widget):
@@ -10,35 +50,34 @@ class TextInput(Widget):
 
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         value: str | None = None,
         readonly: bool = False,
         placeholder: str | None = None,
-        on_change: callable | None = None,
-        on_confirm: callable | None = None,
-        on_gain_focus: callable | None = None,
-        on_lose_focus: callable | None = None,
-        validators: list[callable] | None = None,
+        on_change: toga.widgets.textinput.OnChangeHandler | None = None,
+        on_confirm: OnConfirmHandler | None = None,
+        on_gain_focus: OnGainFocusHandler | None = None,
+        on_lose_focus: OnLoseFocusHandler | None = None,
+        validators: Iterable[Callable[[str], bool]] | None = None,
     ):
         """
         :param id: The ID for the widget.
-        :param style: A style object. If no style is provided, a default style
-            will be applied to the widget.
+        :param style: A style object. If no style is provided, a default style will be
+            applied to the widget.
         :param value: The initial content to display in the widget.
         :param readonly: Can the value of the widget be modified by the user?
-        :param placeholder: The content to display as a placeholder when there
-            is no user content to display.
-        :param on_change: A handler that will be invoked when the value of
-            the widget changes.
-        :param on_confirm: A handler that will be invoked when the user accepts
-            the value of the input (usually by pressing Return on the keyboard).
-        :param on_gain_focus: A handler that will be invoked when the widget
-            gains input focus.
-        :param on_lose_focus: A handler that will be invoked when the widget
-            loses input focus.
-        :param validators: A list of validators to run on the value of the
-            input.
+        :param placeholder: The content to display as a placeholder when there is no
+            user content to display.
+        :param on_change: A handler that will be invoked when the value of the widget
+            changes.
+        :param on_confirm: A handler that will be invoked when the user accepts the
+            value of the input (usually by pressing Return on the keyboard).
+        :param on_gain_focus: A handler that will be invoked when the widget gains
+            input focus.
+        :param on_lose_focus: A handler that will be invoked when the widget loses
+            input focus.
+        :param validators: A list of validators to run on the value of the input.
         """
         super().__init__(id=id, style=style)
 
@@ -64,7 +103,7 @@ def __init__(
         self.on_lose_focus = on_lose_focus
         self.on_gain_focus = on_gain_focus
 
-    def _create(self):
+    def _create(self) -> None:
         self._impl = self.factory.TextInput(interface=self)
 
     @property
@@ -78,7 +117,7 @@ def readonly(self) -> bool:
         return self._impl.get_readonly()
 
     @readonly.setter
-    def readonly(self, value):
+    def readonly(self, value: object) -> None:
         self._impl.set_readonly(bool(value))
 
     @property
@@ -91,7 +130,7 @@ def placeholder(self) -> str:
         return self._impl.get_placeholder()
 
     @placeholder.setter
-    def placeholder(self, value):
+    def placeholder(self, value: object) -> None:
         self._impl.set_placeholder("" if value is None else str(value))
         self.refresh()
 
@@ -109,7 +148,7 @@ def value(self) -> str:
         return self._impl.get_value()
 
     @value.setter
-    def value(self, value: str):
+    def value(self, value: object) -> None:
         if value is None:
             v = ""
         else:
@@ -123,16 +162,16 @@ def is_valid(self) -> bool:
         return self._impl.is_valid()
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """The handler to invoke when the value of the widget changes."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.textinput.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
     @property
-    def validators(self) -> list[callable]:
+    def validators(self) -> list[Callable[[str], bool]]:
         """The list of validators being used to check input on the widget.
 
         Changing the list of validators will cause validation to be performed.
@@ -140,35 +179,35 @@ def validators(self) -> list[callable]:
         return self._validators
 
     @validators.setter
-    def validators(self, validators):
+    def validators(self, validators: Iterable[Callable[[str], bool]] | None) -> None:
         replacing = hasattr(self, "_validators")
         if validators is None:
             self._validators = []
         else:
-            self._validators = validators
+            self._validators = list(validators)
 
         if replacing:
             self._validate()
 
     @property
-    def on_gain_focus(self) -> callable:
+    def on_gain_focus(self) -> OnGainFocusHandler:
         """The handler to invoke when the widget gains input focus."""
         return self._on_gain_focus
 
     @on_gain_focus.setter
-    def on_gain_focus(self, handler):
+    def on_gain_focus(self, handler: OnGainFocusHandler) -> None:
         self._on_gain_focus = wrapped_handler(self, handler)
 
     @property
-    def on_lose_focus(self) -> callable:
+    def on_lose_focus(self) -> OnLoseFocusHandler:
         """The handler to invoke when the widget loses input focus."""
         return self._on_lose_focus
 
     @on_lose_focus.setter
-    def on_lose_focus(self, handler):
+    def on_lose_focus(self, handler: OnLoseFocusHandler) -> None:
         self._on_lose_focus = wrapped_handler(self, handler)
 
-    def _validate(self):
+    def _validate(self) -> None:
         """Validate the current value of the widget.
 
         If a problem is found, the widget will be put into an error state.
@@ -181,18 +220,18 @@ def _validate(self):
         else:
             self._impl.clear_error()
 
-    def _value_changed(self):
+    def _value_changed(self) -> None:
         """Validate the current value of the widget and invoke the on_change handler."""
         self._validate()
         self.on_change()
 
     @property
-    def on_confirm(self) -> callable:
+    def on_confirm(self) -> OnConfirmHandler:
         """The handler to invoke when the user accepts the value of the widget,
         usually by pressing return/enter on the keyboard.
         """
         return self._on_confirm
 
     @on_confirm.setter
-    def on_confirm(self, handler):
+    def on_confirm(self, handler: OnConfirmHandler) -> None:
         self._on_confirm = wrapped_handler(self, handler)
diff --git a/core/src/toga/widgets/timeinput.py b/core/src/toga/widgets/timeinput.py
index f1beee1041..0a23987bb0 100644
--- a/core/src/toga/widgets/timeinput.py
+++ b/core/src/toga/widgets/timeinput.py
@@ -2,21 +2,32 @@
 
 import datetime
 import warnings
+from typing import Any, Protocol
 
+import toga
 from toga.handlers import wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
+
+
+class OnChangeHandler(Protocol):
+    def __call__(self, widget: TimeInput, /, **kwargs: Any) -> object:
+        """A handler to invoke when the time input is changed.
+
+        :param widget: The TimeInput that was changed.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
 
 
 class TimeInput(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         value: datetime.time | None = None,
         min: datetime.time | None = None,
         max: datetime.time | None = None,
-        on_change: callable | None = None,
+        on_change: toga.widgets.timeinput.OnChangeHandler | None = None,
     ):
         """Create a new TimeInput widget.
 
@@ -51,7 +62,18 @@ def value(self) -> datetime.time:
         """
         return self._impl.get_value()
 
-    def _convert_time(self, value):
+    @value.setter
+    def value(self, value: object) -> None:
+        value = self._convert_time(value)
+
+        if value < self.min:
+            value = self.min
+        elif value > self.max:
+            value = self.max
+
+        self._impl.set_value(value)
+
+    def _convert_time(self, value: object) -> datetime.time:
         if value is None:
             value = datetime.datetime.now().time()
         elif isinstance(value, datetime.datetime):
@@ -65,17 +87,6 @@ def _convert_time(self, value):
 
         return value.replace(microsecond=0)
 
-    @value.setter
-    def value(self, value):
-        value = self._convert_time(value)
-
-        if value < self.min:
-            value = self.min
-        elif value > self.max:
-            value = self.max
-
-        self._impl.set_value(value)
-
     @property
     def min(self) -> datetime.time:
         """The minimum allowable time (inclusive). A value of ``None`` will be converted
@@ -87,7 +98,7 @@ def min(self) -> datetime.time:
         return self._impl.get_min_time()
 
     @min.setter
-    def min(self, value):
+    def min(self, value: object) -> None:
         if value is None:
             min = datetime.time(0, 0, 0)
         else:
@@ -110,7 +121,7 @@ def max(self) -> datetime.time:
         return self._impl.get_max_time()
 
     @max.setter
-    def max(self, value):
+    def max(self, value: object) -> None:
         if value is None:
             max = datetime.time(23, 59, 59)
         else:
@@ -123,18 +134,18 @@ def max(self, value):
             self.value = max
 
     @property
-    def on_change(self) -> callable:
+    def on_change(self) -> OnChangeHandler:
         """The handler to invoke when the time value changes."""
         return self._on_change
 
     @on_change.setter
-    def on_change(self, handler):
+    def on_change(self, handler: toga.widgets.timeinput.OnChangeHandler) -> None:
         self._on_change = wrapped_handler(self, handler)
 
 
 # 2023-05: Backwards compatibility
 class TimePicker(TimeInput):
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args: Any, **kwargs: Any):
         warnings.warn("TimePicker has been renamed TimeInput", DeprecationWarning)
 
         for old_name, new_name in [
@@ -155,28 +166,28 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
     @property
-    def min_time(self):
+    def min_time(self) -> datetime.time:
         warnings.warn(
             "TimePicker.min_time has been renamed TimeInput.min", DeprecationWarning
         )
         return self.min
 
     @min_time.setter
-    def min_time(self, value):
+    def min_time(self, value: object) -> None:
         warnings.warn(
             "TimePicker.min_time has been renamed TimeInput.min", DeprecationWarning
         )
         self.min = value
 
     @property
-    def max_time(self):
+    def max_time(self) -> datetime.time:
         warnings.warn(
             "TimePicker.max_time has been renamed TimeInput.max", DeprecationWarning
         )
         return self.max
 
     @max_time.setter
-    def max_time(self, value):
+    def max_time(self, value: object) -> None:
         warnings.warn(
             "TimePicker.max_time has been renamed TimeInput.max", DeprecationWarning
         )
diff --git a/core/src/toga/widgets/tree.py b/core/src/toga/widgets/tree.py
index 55608225b6..4c2b2c616e 100644
--- a/core/src/toga/widgets/tree.py
+++ b/core/src/toga/widgets/tree.py
@@ -1,28 +1,51 @@
 from __future__ import annotations
 
 import warnings
-from typing import Any
+from collections.abc import Iterable
+from typing import Any, Literal, Protocol, TypeVar
 
+import toga
 from toga.handlers import wrapped_handler
 from toga.sources import Node, Source, TreeSource
 from toga.sources.accessors import build_accessors, to_accessor
+from toga.style import Pack
 
 from .base import Widget
 
+SourceT = TypeVar("SourceT", bound=Source)
+
+
+class OnSelectHandler(Protocol):
+    def __call__(self, widget: Tree, /, **kwargs: Any) -> object:
+        """A handler to invoke when the tree is selected.
+
+        :param widget: The Tree that was selected.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
+class OnActivateHandler(Protocol):
+    def __call__(self, widget: Tree, /, **kwargs: Any) -> object:
+        """A handler to invoke when the tree is activated.
+
+        :param widget: The Tree that was activated.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
 
 class Tree(Widget):
     def __init__(
         self,
-        headings: list[str] | None = None,
-        id=None,
-        style=None,
-        data: Any = None,
-        accessors: list[str] | None = None,
+        headings: Iterable[str] | None = None,
+        id: str | None = None,
+        style: Pack | None = None,
+        data: SourceT | object | None = None,
+        accessors: Iterable[str] | None = None,
         multiple_select: bool = False,
-        on_select: callable | None = None,
-        on_activate: callable | None = None,
+        on_select: toga.widgets.tree.OnSelectHandler | None = None,
+        on_activate: toga.widgets.tree.OnActivateHandler | None = None,
         missing_value: str = "",
-        on_double_click=None,  # DEPRECATED
+        on_double_click: None = None,  # DEPRECATED
     ):
         """Create a new Tree widget.
 
@@ -73,12 +96,15 @@ def __init__(
         # End backwards compatibility.
         ######################################################################
 
+        self._headings: list[str] | None
+        self._data: SourceT | TreeSource
+
         if headings is not None:
             self._headings = [heading.split("\n")[0] for heading in headings]
             self._accessors = build_accessors(self._headings, accessors)
         elif accessors is not None:
             self._headings = None
-            self._accessors = accessors
+            self._accessors = list(accessors)
         else:
             raise ValueError(
                 "Cannot create a tree without either headings or accessors"
@@ -98,7 +124,7 @@ def __init__(
         self.on_activate = on_activate
 
     @property
-    def enabled(self) -> bool:
+    def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?
         Tree widgets cannot be disabled; this property will always return True; any
         attempt to modify it will be ignored.
@@ -106,15 +132,15 @@ def enabled(self) -> bool:
         return True
 
     @enabled.setter
-    def enabled(self, value):
+    def enabled(self, value: object) -> None:
         pass
 
-    def focus(self):
-        "No-op; Tree cannot accept input focus"
+    def focus(self) -> None:
+        """No-op; Tree cannot accept input focus."""
         pass
 
     @property
-    def data(self) -> TreeSource:
+    def data(self) -> SourceT | TreeSource:
         """The data to display in the tree.
 
         When setting this property:
@@ -130,7 +156,7 @@ def data(self) -> TreeSource:
         return self._data
 
     @data.setter
-    def data(self, data: Any):
+    def data(self, data: SourceT | object | None) -> None:
         if data is None:
             self._data = TreeSource(accessors=self._accessors, data=[])
         elif isinstance(data, Source):
@@ -159,7 +185,7 @@ def selection(self) -> list[Node] | Node | None:
         """
         return self._impl.get_selection()
 
-    def expand(self, node: Node | None = None):
+    def expand(self, node: Node | None = None) -> None:
         """Expand the specified node of the tree.
 
         If no node is provided, all nodes of the tree will be expanded.
@@ -176,7 +202,7 @@ def expand(self, node: Node | None = None):
         else:
             self._impl.expand_node(node)
 
-    def collapse(self, node: Node | None = None):
+    def collapse(self, node: Node | None = None) -> None:
         """Collapse the specified node of the tree.
 
         If no node is provided, all nodes of the tree will be collapsed.
@@ -191,7 +217,7 @@ def collapse(self, node: Node | None = None):
         else:
             self._impl.collapse_node(node)
 
-    def append_column(self, heading: str, accessor: str | None = None):
+    def append_column(self, heading: str, accessor: str | None = None) -> None:
         """Append a column to the end of the tree.
 
         :param heading: The heading for the new column.
@@ -204,9 +230,9 @@ def append_column(self, heading: str, accessor: str | None = None):
     def insert_column(
         self,
         index: int | str,
-        heading: str | None,
+        heading: str,
         accessor: str | None = None,
-    ):
+    ) -> None:
         """Insert an additional column into the tree.
 
         :param index: The index at which to insert the column, or the accessor of the
@@ -239,7 +265,7 @@ def insert_column(
 
         self._impl.insert_column(index, heading, accessor)
 
-    def remove_column(self, column: int | str):
+    def remove_column(self, column: int | str) -> None:
         """Remove a tree column.
 
         :param column: The index of the column to remove, or the accessor of the column
@@ -261,7 +287,7 @@ def remove_column(self, column: int | str):
         self._impl.remove_column(index)
 
     @property
-    def headings(self) -> list[str]:
+    def headings(self) -> list[str] | None:
         """The column headings for the tree (read-only)"""
         return self._headings
 
@@ -278,22 +304,22 @@ def missing_value(self) -> str:
         return self._missing_value
 
     @property
-    def on_select(self) -> callable:
+    def on_select(self) -> OnSelectHandler:
         """The callback function that is invoked when a row of the tree is selected."""
         return self._on_select
 
     @on_select.setter
-    def on_select(self, handler: callable):
+    def on_select(self, handler: toga.widgets.tree.OnSelectHandler) -> None:
         self._on_select = wrapped_handler(self, handler)
 
     @property
-    def on_activate(self) -> callable:
+    def on_activate(self) -> OnActivateHandler:
         """The callback function that is invoked when a row of the tree is activated,
         usually with a double click or similar action."""
         return self._on_activate
 
     @on_activate.setter
-    def on_activate(self, handler):
+    def on_activate(self, handler: toga.widgets.tree.OnActivateHandler) -> None:
         self._on_activate = wrapped_handler(self, handler)
 
     ######################################################################
@@ -301,7 +327,7 @@ def on_activate(self, handler):
     ######################################################################
 
     @property
-    def on_double_click(self):
+    def on_double_click(self) -> OnActivateHandler:
         """**DEPRECATED**: Use ``on_activate``"""
         warnings.warn(
             "Tree.on_double_click has been renamed Tree.on_activate.",
@@ -310,7 +336,7 @@ def on_double_click(self):
         return self.on_activate
 
     @on_double_click.setter
-    def on_double_click(self, handler):
+    def on_double_click(self, handler: toga.widgets.tree.OnActivateHandler) -> None:
         warnings.warn(
             "Tree.on_double_click has been renamed Tree.on_activate.",
             DeprecationWarning,
diff --git a/core/src/toga/widgets/webview.py b/core/src/toga/widgets/webview.py
index f2282efe73..740d830af2 100644
--- a/core/src/toga/widgets/webview.py
+++ b/core/src/toga/widgets/webview.py
@@ -1,24 +1,34 @@
 from __future__ import annotations
 
 import asyncio
+from typing import Any, Protocol
 
-from toga.handlers import AsyncResult, wrapped_handler
+from toga.handlers import AsyncResult, OnResultT, wrapped_handler
 
-from .base import Widget
+from .base import StyleT, Widget
 
 
 class JavaScriptResult(AsyncResult):
     RESULT_TYPE = "JavaScript"
 
 
+class OnWebViewLoadHandler(Protocol):
+    def __call__(self, widget: WebView, /, **kwargs: Any) -> object:
+        """A handler to invoke when the WebView is loaded.
+
+        :param widget: The WebView that was loaded.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        """
+
+
 class WebView(Widget):
     def __init__(
         self,
-        id=None,
-        style=None,
+        id: str | None = None,
+        style: StyleT | None = None,
         url: str | None = None,
         user_agent: str | None = None,
-        on_webview_load: callable | None = None,
+        on_webview_load: OnWebViewLoadHandler | None = None,
     ):
         """Create a new WebView widget.
 
@@ -32,7 +42,6 @@ def __init__(
         :param on_webview_load: A handler that will be invoked when the web view
             finishes loading.
         """
-
         super().__init__(id=id, style=style)
 
         self._impl = self.factory.WebView(interface=self)
@@ -42,11 +51,9 @@ def __init__(
         self.on_webview_load = on_webview_load
         self.url = url
 
-    def _set_url(self, url, future):
+    def _set_url(self, url: str | None, future: asyncio.Future | None) -> None:
         # Utility method for validating and setting the URL with a future.
-        if (url is not None) and not (
-            url.startswith("https://") or url.startswith("http://")
-        ):
+        if (url is not None) and not url.startswith(("https://", "http://")):
             raise ValueError("WebView can only display http:// and https:// URLs")
 
         self._impl.set_url(url, future=future)
@@ -62,10 +69,10 @@ def url(self) -> str | None:
         return self._impl.get_url()
 
     @url.setter
-    def url(self, value):
+    def url(self, value: str | None) -> None:
         self._set_url(value, future=None)
 
-    async def load_url(self, url: str):
+    async def load_url(self, url: str) -> asyncio.Future:
         """Load a URL, and wait until the next :any:`on_webview_load` event.
 
         **Note:** On Android, this method will return immediately.
@@ -78,7 +85,7 @@ async def load_url(self, url: str):
         return await loaded_future
 
     @property
-    def on_webview_load(self) -> callable:
+    def on_webview_load(self) -> OnWebViewLoadHandler:
         """The handler to invoke when the web view finishes loading.
 
         Rendering web content is a complex, multi-threaded process. Although a page
@@ -95,7 +102,7 @@ def on_webview_load(self) -> callable:
         return self._on_webview_load
 
     @on_webview_load.setter
-    def on_webview_load(self, handler):
+    def on_webview_load(self, handler: OnWebViewLoadHandler) -> None:
         if handler and not getattr(self._impl, "SUPPORTS_ON_WEBVIEW_LOAD", True):
             self.factory.not_implemented("WebView.on_webview_load")
 
@@ -111,10 +118,10 @@ def user_agent(self) -> str:
         return self._impl.get_user_agent()
 
     @user_agent.setter
-    def user_agent(self, value):
+    def user_agent(self, value: str) -> None:
         self._impl.set_user_agent(value)
 
-    def set_content(self, root_url: str, content: str):
+    def set_content(self, root_url: str, content: str) -> None:
         """Set the HTML content of the WebView.
 
         **Note:** On Android and Windows, the ``root_url`` argument is ignored. Calling
@@ -126,7 +133,11 @@ def set_content(self, root_url: str, content: str):
         """
         self._impl.set_content(root_url, content)
 
-    def evaluate_javascript(self, javascript, on_result=None) -> JavaScriptResult:
+    def evaluate_javascript(
+        self,
+        javascript: str,
+        on_result: OnResultT | None = None,
+    ) -> JavaScriptResult:
         """Evaluate a JavaScript expression.
 
         **This is an asynchronous method**. There is no guarantee that the JavaScript
diff --git a/core/src/toga/window.py b/core/src/toga/window.py
index 651068d6b5..725e5c4a9e 100644
--- a/core/src/toga/window.py
+++ b/core/src/toga/window.py
@@ -2,22 +2,18 @@
 
 import warnings
 from builtins import id as identifier
-from collections.abc import Mapping, MutableSet
+from collections.abc import Iterator
 from pathlib import Path
 from typing import (
     TYPE_CHECKING,
     Any,
-    ItemsView,
-    Iterator,
-    KeysView,
     Literal,
     Protocol,
     TypeVar,
-    ValuesView,
     overload,
 )
 
-from toga.command import Command, CommandSet
+from toga.command import CommandSet
 from toga.handlers import AsyncResult, wrapped_handler
 from toga.images import Image
 from toga.platform import get_platform_factory
@@ -35,7 +31,7 @@ class FilteredWidgetRegistry:
     # A class that exposes a mapping lookup interface, filtered to widgets from a single
     # window. The underlying data store is on the app.
 
-    def __init__(self, window):
+    def __init__(self, window: Window) -> None:
         self._window = window
 
     def __len__(self) -> int:
@@ -60,51 +56,49 @@ def __iter__(self) -> Iterator[Widget]:
     def __repr__(self) -> str:
         return "{" + ", ".join(f"{k!r}: {v!r}" for k, v in sorted(self.items())) + "}"
 
-    def items(self) -> ItemsView:
+    def items(self) -> Iterator[tuple[str, Widget]]:
         for item in self._window.app.widgets.items():
             if item[1].window == self._window:
                 yield item
 
-    def keys(self) -> KeysView:
+    def keys(self) -> Iterator[str]:
         for item in self._window.app.widgets.items():
             if item[1].window == self._window:
                 yield item[0]
 
-    def values(self) -> ValuesView:
+    def values(self) -> Iterator[Widget]:
         for item in self._window.app.widgets.items():
             if item[1].window == self._window:
                 yield item[1]
 
 
 class OnCloseHandler(Protocol):
-    def __call__(self, window: Window, **kwargs: Any) -> bool:
+    def __call__(self, window: Window, /, **kwargs: Any) -> bool:
         """A handler to invoke when a window is about to close.
 
         The return value of this callback controls whether the window is allowed to close.
         This can be used to prevent a window closing with unsaved changes, etc.
 
         :param window: The window instance that is closing.
-        :param kwargs: Ensures compatibility with additional arguments introduced in
-            future versions.
-        :returns: ``True`` if the window is allowed to close; ``False`` if the window is not
-            allowed to close.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
+        :returns: ``True`` if the window is allowed to close; ``False`` if the window
+            is not allowed to close.
         """
-        ...
 
 
-T = TypeVar("T")
+_DialogResultT = TypeVar("_DialogResultT")
 
 
-class DialogResultHandler(Protocol[T]):
-    def __call__(self, window: Window, result: T, **kwargs: Any) -> None:
+class DialogResultHandler(Protocol[_DialogResultT]):
+    def __call__(
+        self, window: Window, result: _DialogResultT, /, **kwargs: Any
+    ) -> object:
         """A handler to invoke when a dialog is closed.
 
         :param window: The window that opened the dialog.
+        :param kwargs: Ensures compatibility with arguments added in future versions.
         :param result: The result returned by the dialog.
-        :param kwargs: Ensures compatibility with additional arguments introduced in
-            future versions.
         """
-        ...
 
 
 class Dialog(AsyncResult):
@@ -130,8 +124,8 @@ def __init__(
         minimizable: bool = True,
         on_close: OnCloseHandler | None = None,
         content: Widget | None = None,
-        resizeable=None,  # DEPRECATED
-        closeable=None,  # DEPRECATED
+        resizeable: None = None,  # DEPRECATED
+        closeable: None = None,  # DEPRECATED
     ) -> None:
         """Create a new Window.
 
@@ -174,8 +168,8 @@ def __init__(
         from toga import App
 
         self._id = str(id if id else identifier(self))
-        self._impl = None
-        self._content = None
+        self._impl: Any = None
+        self._content: Widget | None = None
         self._is_full_screen = False
         self._closed = False
 
@@ -194,7 +188,8 @@ def __init__(
         )
 
         # Add the window to the app
-        self._app = None
+        # _app will only be None until the window is added to the app below
+        self._app: App = None
         if App.app is None:
             raise RuntimeError("Cannot create a Window before creating an App")
         App.app.windows.add(self)
@@ -208,7 +203,7 @@ def __init__(
 
         self.on_close = on_close
 
-    def __lt__(self, other) -> bool:
+    def __lt__(self, other: Window) -> bool:
         return self.id < other.id
 
     ######################################################################
@@ -330,12 +325,12 @@ def content(self, widget: Widget) -> None:
         widget.refresh()
 
     @property
-    def toolbar(self) -> MutableSet[Command]:
+    def toolbar(self) -> CommandSet:
         """Toolbar for the window."""
         return self._toolbar
 
     @property
-    def widgets(self) -> Mapping[str, Widget]:
+    def widgets(self) -> FilteredWidgetRegistry:
         """The widgets contained in the window.
 
         Can be used to look up widgets by ID (e.g., ``window.widgets["my_id"]``).
@@ -464,7 +459,7 @@ def as_image(self, format: type[ImageT] = Image) -> ImageT:
     ######################################################################
 
     @property
-    def on_close(self) -> OnCloseHandler:
+    def on_close(self) -> OnCloseHandler | None:
         """The handler to invoke if the user attempts to close the window."""
         return self._on_close
 
@@ -605,7 +600,7 @@ def stack_trace_dialog(
         title: str,
         message: str,
         content: str,
-        retry: Literal[True] = False,
+        retry: Literal[True] = True,
         on_result: DialogResultHandler[bool] | None = None,
     ) -> Dialog: ...
 
@@ -616,7 +611,7 @@ def stack_trace_dialog(
         message: str,
         content: str,
         retry: bool = False,
-        on_result: DialogResultHandler[bool | None] | None = None,
+        on_result: DialogResultHandler[bool] | DialogResultHandler[None] | None = None,
     ) -> Dialog: ...
 
     def stack_trace_dialog(
@@ -625,7 +620,7 @@ def stack_trace_dialog(
         message: str,
         content: str,
         retry: bool = False,
-        on_result: DialogResultHandler[bool | None] | None = None,
+        on_result: DialogResultHandler[bool] | DialogResultHandler[None] | None = None,
     ) -> Dialog:
         """Open a dialog to display a large block of text, such as a stack trace.
 
@@ -687,7 +682,7 @@ def save_file_dialog(
         # Convert suggested filename to a path (if it isn't already),
         # and break it into a filename and a directory
         suggested_path = Path(suggested_filename)
-        initial_directory = suggested_path.parent
+        initial_directory: Path | None = suggested_path.parent
         if initial_directory == Path("."):
             initial_directory = None
         filename = suggested_path.name
@@ -708,8 +703,8 @@ def open_file_dialog(
         initial_directory: Path | str | None = None,
         file_types: list[str] | None = None,
         multiple_select: Literal[False] = False,
-        on_result: DialogResultHandler[Path | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: DialogResultHandler[Path] | DialogResultHandler[None] | None = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog: ...
 
     @overload
@@ -719,8 +714,10 @@ def open_file_dialog(
         initial_directory: Path | str | None = None,
         file_types: list[str] | None = None,
         multiple_select: Literal[True] = True,
-        on_result: DialogResultHandler[list[Path] | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: (
+            DialogResultHandler[list[Path]] | DialogResultHandler[None] | None
+        ) = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog: ...
 
     @overload
@@ -730,8 +727,13 @@ def open_file_dialog(
         initial_directory: Path | str | None = None,
         file_types: list[str] | None = None,
         multiple_select: bool = False,
-        on_result: DialogResultHandler[list[Path] | Path | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: (
+            DialogResultHandler[list[Path]]
+            | DialogResultHandler[Path]
+            | DialogResultHandler[None]
+            | None
+        ) = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog: ...
 
     def open_file_dialog(
@@ -740,8 +742,13 @@ def open_file_dialog(
         initial_directory: Path | str | None = None,
         file_types: list[str] | None = None,
         multiple_select: bool = False,
-        on_result: DialogResultHandler[list[Path] | Path | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: (
+            DialogResultHandler[list[Path]]
+            | DialogResultHandler[Path]
+            | DialogResultHandler[None]
+            | None
+        ) = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog:
         """Prompt the user to select a file (or files) to open.
 
@@ -797,8 +804,8 @@ def select_folder_dialog(
         title: str,
         initial_directory: Path | str | None = None,
         multiple_select: Literal[False] = False,
-        on_result: DialogResultHandler[Path | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: DialogResultHandler[Path] | DialogResultHandler[None] | None = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog: ...
 
     @overload
@@ -807,8 +814,10 @@ def select_folder_dialog(
         title: str,
         initial_directory: Path | str | None = None,
         multiple_select: Literal[True] = True,
-        on_result: DialogResultHandler[list[Path] | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: (
+            DialogResultHandler[list[Path]] | DialogResultHandler[None] | None
+        ) = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog: ...
 
     @overload
@@ -817,8 +826,13 @@ def select_folder_dialog(
         title: str,
         initial_directory: Path | str | None = None,
         multiple_select: bool = False,
-        on_result: DialogResultHandler[list[Path] | Path | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: (
+            DialogResultHandler[list[Path]]
+            | DialogResultHandler[Path]
+            | DialogResultHandler[None]
+            | None
+        ) = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog: ...
 
     def select_folder_dialog(
@@ -826,8 +840,13 @@ def select_folder_dialog(
         title: str,
         initial_directory: Path | str | None = None,
         multiple_select: bool = False,
-        on_result: DialogResultHandler[list[Path] | Path | None] | None = None,
-        multiselect=None,  # DEPRECATED
+        on_result: (
+            DialogResultHandler[list[Path]]
+            | DialogResultHandler[Path]
+            | DialogResultHandler[None]
+            | None
+        ) = None,
+        multiselect: None = None,  # DEPRECATED
     ) -> Dialog:
         """Prompt the user to select a directory (or directories).
 
@@ -878,7 +897,6 @@ def select_folder_dialog(
     ######################################################################
     # 2023-08: Backwards compatibility
     ######################################################################
-
     @property
     def resizeable(self) -> bool:
         """**DEPRECATED** Use :attr:`resizable`"""
@@ -896,3 +914,7 @@ def closeable(self) -> bool:
             DeprecationWarning,
         )
         return self._closable
+
+    ######################################################################
+    # End Backwards compatibility
+    ######################################################################
diff --git a/docs/reference/api/containers/optioncontainer.rst b/docs/reference/api/containers/optioncontainer.rst
index 284d584c69..b6e3d79a0e 100644
--- a/docs/reference/api/containers/optioncontainer.rst
+++ b/docs/reference/api/containers/optioncontainer.rst
@@ -70,7 +70,7 @@ of content can be modified after initial construction:
     icecream = toga.Box()
     container.content.append("Ice Cream", icecream, toga.Icon("icecream"))
 
-OptionContainer content can also be specified by using :any:`OptionItem` instances
+OptionContainer content can also be specified by using :any:`toga.OptionItem` instances
 instead of tuples. This enables you to be explicit when setting an icon or enabled
 status; it also allows you to set the initial enabled status *without* setting an icon:
 
@@ -119,7 +119,7 @@ item, you can specify an item using:
       # Delete tab labeled "Pasta"
       del container.content["Pasta"]
 
-* A reference to an :any:`OptionItem`:
+* A reference to an :any:`toga.OptionItem`:
 
   .. code-block:: python
 
@@ -174,16 +174,16 @@ Notes
 Reference
 ---------
 
-.. c:type:: OptionContainerContent
+.. c:type:: OptionContainerContentT
 
     An item of :any:`OptionContainer` content can be:
 
     * a 2-tuple, containing the title for the tab, and the content widget;
-    * a 3-tuple, containing the title, content widget, and :any:`icon <IconContent>`
+    * a 3-tuple, containing the title, content widget, and :any:`icon <IconContentT>`
       for the tab;
-    * a 4-tuple, containing the title, content widget, :any:`icon <IconContent>` for
+    * a 4-tuple, containing the title, content widget, :any:`icon <IconContentT>` for
       the tab, and enabled status; or
-    * an :any:`OptionItem` instance.
+    * an :any:`toga.OptionItem` instance.
 
 .. autoclass:: toga.OptionContainer
    :exclude-members: app, window
@@ -193,4 +193,6 @@ Reference
 .. autoclass:: toga.widgets.optioncontainer.OptionList
     :special-members: __getitem__, __delitem__
 
+.. autoclass:: toga.widgets.optioncontainer.OptionItem
+
 .. autoprotocol:: toga.widgets.optioncontainer.OnSelectHandler
diff --git a/docs/reference/api/containers/scrollcontainer.rst b/docs/reference/api/containers/scrollcontainer.rst
index cb068354f1..53020130d2 100644
--- a/docs/reference/api/containers/scrollcontainer.rst
+++ b/docs/reference/api/containers/scrollcontainer.rst
@@ -60,3 +60,5 @@ Reference
 
 .. autoclass:: toga.ScrollContainer
    :exclude-members: window, app
+
+.. autoprotocol:: toga.widgets.scrollcontainer.OnScrollHandler
diff --git a/docs/reference/api/containers/splitcontainer.rst b/docs/reference/api/containers/splitcontainer.rst
index 48c0ea8e13..6d04683daf 100644
--- a/docs/reference/api/containers/splitcontainer.rst
+++ b/docs/reference/api/containers/splitcontainer.rst
@@ -49,7 +49,7 @@ Usage
     left_container = toga.Box()
     right_container = toga.ScrollContainer()
 
-    split = toga.SplitContainer(content=[left_container, right_container])
+    split = toga.SplitContainer(content=(left_container, right_container))
 
 Content can be specified when creating the widget, or after creation by assigning
 the ``content`` attribute. The direction of the split can also be configured, either
@@ -65,7 +65,7 @@ at time of creation, or by setting the ``direction`` attribute:
     left_container = toga.Box()
     right_container = toga.ScrollContainer()
 
-    split.content = [left_container, right_container]
+    split.content = (left_container, right_container)
 
 By default, the space of the SplitContainer will be evenly divided between the
 two panels. To specify an uneven split, you can provide a flex value when specifying
@@ -80,7 +80,7 @@ and right panels.
     left_container = toga.Box()
     right_container = toga.ScrollContainer()
 
-    split.content = [(left_container, 3), (right_container, 2)]
+    split.content = ((left_container, 3), (right_container, 2))
 
 This only specifies the initial split; the split can be modified by the user
 once it is displayed.
@@ -88,5 +88,12 @@ once it is displayed.
 Reference
 ---------
 
+.. c:type:: SplitContainerContentT
+
+    An item of :any:`SplitContainer` content can be:
+
+    * a :class:`~toga.Widget`; or
+    * a 2-tuple, containing a :class:`~toga.Widget`, and an :any:`int` flex value
+
 .. autoclass:: toga.SplitContainer
    :exclude-members: HORIZONTAL, VERTICAL, window, app
diff --git a/docs/reference/api/resources/icons.rst b/docs/reference/api/resources/icons.rst
index 90cdfad228..3f517d4ef7 100644
--- a/docs/reference/api/resources/icons.rst
+++ b/docs/reference/api/resources/icons.rst
@@ -80,7 +80,7 @@ permission error). In this case, an error will be raised.
 Reference
 ---------
 
-.. c:type:: IconContent
+.. c:type:: IconContentT
 
     When specifying an :any:`Icon`, you can provide:
 
diff --git a/docs/reference/api/resources/images.rst b/docs/reference/api/resources/images.rst
index 59aa204451..498d2ccee8 100644
--- a/docs/reference/api/resources/images.rst
+++ b/docs/reference/api/resources/images.rst
@@ -30,7 +30,7 @@ Usage
     widget configured to use an Icon (probably :class:`~toga.Button`), *not* an
     ``on_press`` handler on an :class:`~toga.Image` or :class:`~toga.ImageView`.
 
-An image can be constructed from a :any:`wide range of sources <ImageContent>`:
+An image can be constructed from a :any:`wide range of sources <ImageContentT>`:
 
 .. code-block:: python
 
@@ -86,13 +86,13 @@ Notes
 Reference
 ---------
 
-.. c:type:: ImageContent
+.. c:type:: ImageContentT
 
     When specifying content for an :any:`Image`, you can provide:
 
     * a string specifying an absolute or relative path to a file in a :ref:`known image
       format <known-image-formats>`;
-    * an absolute or relative :any:`pathlib.Path` object describing a file in a
+    * an absolute or relative :class:`~pathlib.Path` object describing a file in a
       :ref:`known image format <known-image-formats>`;
     * a "blob of bytes" data type (:any:`bytes`, :any:`bytearray`, or :any:`memoryview`)
       containing raw image data in a :ref:`known image format <known-image-formats>`;
diff --git a/docs/reference/api/widgets/dateinput.rst b/docs/reference/api/widgets/dateinput.rst
index 2854869094..c0da1d954d 100644
--- a/docs/reference/api/widgets/dateinput.rst
+++ b/docs/reference/api/widgets/dateinput.rst
@@ -60,3 +60,5 @@ Reference
 ---------
 
 .. autoclass:: toga.DateInput
+
+.. autoprotocol:: toga.widgets.dateinput.OnChangeHandler
diff --git a/docs/reference/api/widgets/detailedlist.rst b/docs/reference/api/widgets/detailedlist.rst
index 55693e2bb2..306d858e58 100644
--- a/docs/reference/api/widgets/detailedlist.rst
+++ b/docs/reference/api/widgets/detailedlist.rst
@@ -146,3 +146,8 @@ Reference
 ---------
 
 .. autoclass:: toga.DetailedList
+
+.. autoprotocol:: toga.widgets.detailedlist.OnPrimaryActionHandler
+.. autoprotocol:: toga.widgets.detailedlist.OnSecondaryActionHandler
+.. autoprotocol:: toga.widgets.detailedlist.OnRefreshHandler
+.. autoprotocol:: toga.widgets.detailedlist.OnSelectHandler
diff --git a/docs/reference/api/widgets/multilinetextinput.rst b/docs/reference/api/widgets/multilinetextinput.rst
index 35fbe4f176..8c69cd4cd2 100644
--- a/docs/reference/api/widgets/multilinetextinput.rst
+++ b/docs/reference/api/widgets/multilinetextinput.rst
@@ -71,3 +71,5 @@ Reference
 ---------
 
 .. autoclass:: toga.MultilineTextInput
+
+.. autoprotocol:: toga.widgets.multilinetextinput.OnChangeHandler
diff --git a/docs/reference/api/widgets/numberinput.rst b/docs/reference/api/widgets/numberinput.rst
index 68bde9474d..77b532a693 100644
--- a/docs/reference/api/widgets/numberinput.rst
+++ b/docs/reference/api/widgets/numberinput.rst
@@ -53,11 +53,13 @@ Usage
     widget = toga.NumberInput(min_value=1, max_value=10, step=0.001)
     widget.value = 2.718
 
-NumberInput's properties can accept integers, floats, and strings containing
-numbers, but they always return :any:`decimal.Decimal` objects to ensure
-precision is retained.
+NumberInput's properties can accept :class:`~decimal.Decimal`, :any:`int`, :any:`float`,
+or :any:`str` containing numbers, but they always return :class:`~decimal.Decimal`
+objects to ensure precision is retained.
 
 Reference
 ---------
 
 .. autoclass:: toga.NumberInput
+
+.. autoprotocol:: toga.widgets.numberinput.OnChangeHandler
diff --git a/docs/reference/api/widgets/slider.rst b/docs/reference/api/widgets/slider.rst
index a2569d5bdb..c021103806 100644
--- a/docs/reference/api/widgets/slider.rst
+++ b/docs/reference/api/widgets/slider.rst
@@ -69,3 +69,7 @@ Reference
 ---------
 
 .. autoclass:: toga.Slider
+
+.. autoprotocol:: toga.widgets.slider.OnChangeHandler
+.. autoprotocol:: toga.widgets.slider.OnPressHandler
+.. autoprotocol:: toga.widgets.slider.OnReleaseHandler
diff --git a/docs/reference/api/widgets/switch.rst b/docs/reference/api/widgets/switch.rst
index d4081b9141..38d8d87e74 100644
--- a/docs/reference/api/widgets/switch.rst
+++ b/docs/reference/api/widgets/switch.rst
@@ -84,3 +84,5 @@ Reference
 ---------
 
 .. autoclass:: toga.Switch
+
+.. autoprotocol:: toga.widgets.switch.OnChangeHandler
diff --git a/docs/reference/api/widgets/table.rst b/docs/reference/api/widgets/table.rst
index 66f6c8dfbd..a910c688d9 100644
--- a/docs/reference/api/widgets/table.rst
+++ b/docs/reference/api/widgets/table.rst
@@ -135,3 +135,6 @@ Reference
 ---------
 
 .. autoclass:: toga.Table
+
+.. autoprotocol:: toga.widgets.table.OnSelectHandler
+.. autoprotocol:: toga.widgets.table.OnActivateHandler
diff --git a/docs/reference/api/widgets/textinput.rst b/docs/reference/api/widgets/textinput.rst
index f95e64d242..49512eb092 100644
--- a/docs/reference/api/widgets/textinput.rst
+++ b/docs/reference/api/widgets/textinput.rst
@@ -94,3 +94,8 @@ Reference
 ---------
 
 .. autoclass:: toga.TextInput
+
+.. autoprotocol:: toga.widgets.textinput.OnChangeHandler
+.. autoprotocol:: toga.widgets.textinput.OnConfirmHandler
+.. autoprotocol:: toga.widgets.textinput.OnGainFocusHandler
+.. autoprotocol:: toga.widgets.textinput.OnLoseFocusHandler
diff --git a/docs/reference/api/widgets/timeinput.rst b/docs/reference/api/widgets/timeinput.rst
index c6d8527b72..1218cc4f45 100644
--- a/docs/reference/api/widgets/timeinput.rst
+++ b/docs/reference/api/widgets/timeinput.rst
@@ -64,3 +64,5 @@ Reference
 ---------
 
 .. autoclass:: toga.TimeInput
+
+.. autoprotocol:: toga.widgets.timeinput.OnChangeHandler
diff --git a/docs/reference/api/widgets/tree.rst b/docs/reference/api/widgets/tree.rst
index 9bb05f5706..cba03f43e1 100644
--- a/docs/reference/api/widgets/tree.rst
+++ b/docs/reference/api/widgets/tree.rst
@@ -152,3 +152,6 @@ Reference
 ---------
 
 .. autoclass:: toga.Tree
+
+.. autoprotocol:: toga.widgets.tree.OnSelectHandler
+.. autoprotocol:: toga.widgets.tree.OnActivateHandler
diff --git a/docs/reference/api/widgets/webview.rst b/docs/reference/api/widgets/webview.rst
index 0927cfc2f2..ccc035fa9d 100644
--- a/docs/reference/api/widgets/webview.rst
+++ b/docs/reference/api/widgets/webview.rst
@@ -109,3 +109,5 @@ Reference
 ---------
 
 .. autoclass:: toga.WebView
+
+.. autoprotocol:: toga.widgets.webview.OnWebViewLoadHandler
diff --git a/docs/reference/api/window.rst b/docs/reference/api/window.rst
index 2f5cb2a368..6cf24d63b1 100644
--- a/docs/reference/api/window.rst
+++ b/docs/reference/api/window.rst
@@ -42,14 +42,14 @@ An operating system-managed container of widgets.
 Usage
 -----
 
-A window is the top-level container that the operating system uses to display widgets. A
-window may also have other decorations, such as a title bar or toolbar.
+A window is the top-level container that the operating system uses to display widgets.
+A window may also have other decorations, such as a title bar or toolbar.
 
 When first created, a window is not visible. To display it, call the
 :meth:`~toga.Window.show` method.
 
 The window has content, which will usually be a container widget of some kind. The
-content of the window can be changed by re-assigning its `content` attribute to a
+content of the window can be changed by re-assigning its ``content`` attribute to a
 different widget.
 
 .. code-block:: python
@@ -96,5 +96,6 @@ Reference
 
 .. autoclass:: toga.Window
 
+.. autoprotocol:: toga.window.Dialog
 .. autoprotocol:: toga.window.OnCloseHandler
 .. autoprotocol:: toga.window.DialogResultHandler
diff --git a/docs/spelling_wordlist b/docs/spelling_wordlist
index aa4837f1be..6992245f2c 100644
--- a/docs/spelling_wordlist
+++ b/docs/spelling_wordlist
@@ -3,6 +3,7 @@ accessors
 amongst
 App
 apps
+Async
 awaitable
 backend
 backends
diff --git a/dummy/src/toga_dummy/plugins/image_formats.py b/dummy/src/toga_dummy/plugins/image_formats.py
index d58686b739..923ceb418c 100644
--- a/dummy/src/toga_dummy/plugins/image_formats.py
+++ b/dummy/src/toga_dummy/plugins/image_formats.py
@@ -6,7 +6,7 @@
 import toga
 
 if TYPE_CHECKING:
-    from toga.images import BytesLike
+    from toga.images import BytesLikeT
 
 
 class CustomImage:
@@ -26,7 +26,7 @@ def convert_from_format(image_in_format: CustomImage):
 
     @staticmethod
     def convert_to_format(
-        data: BytesLike,
+        data: BytesLikeT,
         image_class: type[CustomImage],
     ) -> CustomImage:
         image = image_class()
@@ -45,7 +45,7 @@ def convert_from_format(image_in_format: Any):
 
     @staticmethod
     def convert_to_format(
-        data: BytesLike,
+        data: BytesLikeT,
         image_class: type[Any],
     ) -> Any:
         raise Exception("Converter should be disabled")