feat: computed should be glitch free

This is only a failing test for now to demonstrate what can go wrong.
1) without an error, the system can see an invalid state
2) an invalid state can also trigger an exception

See https://dev.to/this-is-learning/derivations-in-reactivity-4fo1
for the loose definition of glitch-free

tests/unit/toestand_test.py

@@ -1233,3 +1233,59 @@ class Person(BaseModel):
     assert person.get().height == 2.0
     assert Ref(person.fields.name).get() == "Maria"
     assert Ref(person.fields.height).get() == 2.0
+@dataclasses.dataclass(frozen=True)
+class Profile:
+    name: str
+    surname: str
+
+
+def test_computed_glitch_invalid_state_without_error():
+    from solara.toestand import Computed
+
+    profile = Reactive(Profile(name="John", surname="Doe"))
+
+    name = Computed(lambda: profile.value.name)
+    surname = Computed(lambda: profile.value.surname)
+
+    computed_initials = []
+
+    def compute_initials():
+        initials = name.value[0] + surname.value[0]
+        computed_initials.append(initials)
+        return initials
+
+    initials = Computed(compute_initials)
+
+    assert name.value == "John"
+    assert surname.value == "Doe"
+    assert initials.value == "JD"
+    assert computed_initials == ["JD"]
+
+    profile.value = Profile(name="Rosa", surname="Breddels")
+
+    assert name.value == "Rosa"
+    assert surname.value == "Breddels"
+    assert initials.value == "RB"
+    assert computed_initials == ["JD", "RB"]
+
+
+@dataclasses.dataclass(frozen=True)
+class CountrySelection:
+    countries: List[str]
+    selected: str
+
+
+def test_computed_glitch_invalid_state_with_error():
+    from solara.toestand import Computed
+
+    country_selection = Reactive(CountrySelection(countries=["Netherlands", "Belgium", "Germany"], selected="Germany"))
+
+    countries = Computed(lambda: country_selection.value.countries)
+    selected = Computed(lambda: country_selection.value.selected)
+    selected_index = Computed(lambda: countries.value.index(selected.value))
+
+    assert selected_index.value == 2
+
+    country_selection.value = CountrySelection(countries=["China", "Japan"], selected="Japan")
+
+    assert selected_index.value == 1