[android] improve performance of Shell flyout opening

Context: dotnet#10713
Context: https://github.com/supershopping/ShellFlyoutLagging

In reviewing the above customer sample, there is a noticeable delay on
some devices when you open a Shell flyout for the first time.

This was pretty close to a default Shell setup:

    <Shell Shell.FlyoutBehavior="Flyout">
        <FlyoutItem Title="Main Page">
            <ShellContent Title="Main Page"  Route="MainPage" ContentTemplate="{DataTemplate local:MainPage}" />
        </FlyoutItem>
        <FlyoutItem Title="Test Page">
            <ShellContent Title="Test Page"  Route="TestPage" ContentTemplate="{DataTemplate local:TestPage}" />
        </FlyoutItem>
    </Shell>

Reviewing the code, there was an oddity where we create & replace
a `RecyclerView`'s `LayoutManager`:

    SetLayoutManager(_layoutManager = new ScrollLayoutManager(context, (int)Orientation.Vertical, false));
    SetLayoutManager(new LinearLayoutManager(context, (int)Orientation.Vertical, false));

We think this is likely something that went wrong during a git merge, etc.

In addition to removing code, I create a new `ShellRecyclerView` in Java
that reduces the amount of interop calls from C# to Java. We can do
all the work now in the `ShellRecyclerView`'s constructor:

    var adapter = new ShellFlyoutRecyclerAdapter(ShellContext, OnElementSelected);
    var recyclerView = new ShellRecyclerView(context, adapter);

`dotnet-trace` output shows these changes have a reasonable impact on
a Pixel 5 device:

    Before:
    35.34ms microsoft.maui.controls!Microsoft.Maui.Controls.Platform.Compatibility.ShellFlyoutTemplatedContentRenderer.CreateFlyoutContent()
    After:
    17.64ms microsoft.maui.controls!Microsoft.Maui.Controls.Platform.Compatibility.ShellFlyoutTemplatedContentRenderer.CreateFlyoutContent()

Saving about ~17.7ms of time opening the Shell flyout on Android.

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellFlyoutTemplatedContentRenderer.cs

@@ -200,8 +200,8 @@ protected virtual void UpdateFlyoutContent()
 		void DisconnectRecyclerView()
 		{
 			if (_flyoutContentView.IsAlive() &&
-				_flyoutContentView is RecyclerViewContainer rvc &&
-				rvc.GetAdapter() is ShellFlyoutRecyclerAdapter sfra)
+				_flyoutContentView is ShellRecyclerView srv &&
+				srv.GetAdapter() is ShellFlyoutRecyclerAdapter sfra)
 			{
 				sfra.Disconnect();
 			}
@@ -219,29 +219,17 @@ AView CreateFlyoutContent(ViewGroup rootView)
 
 			DisconnectRecyclerView();
 
+			var context = ShellContext.AndroidContext;
 			var content = ((IShellController)ShellContext.Shell).FlyoutContent;
 			if (content == null)
 			{
-				var lp = new CoordinatorLayout.LayoutParams(CoordinatorLayout.LayoutParams.MatchParent, CoordinatorLayout.LayoutParams.MatchParent);
-				lp.Behavior = new AppBarLayout.ScrollingViewBehavior();
-				var context = ShellContext.AndroidContext;
-				var recyclerView = new RecyclerViewContainer(context)
-				{
-					LayoutParameters = lp
-				};
-
-				recyclerView.SetAdapter(new ShellFlyoutRecyclerAdapter(ShellContext, OnElementSelected));
-
+				var adapter = new ShellFlyoutRecyclerAdapter(ShellContext, OnElementSelected);
+				var recyclerView = new ShellRecyclerView(context, adapter);
 				return recyclerView;
 			}
 
-			_contentView = new ShellViewRenderer(ShellContext.AndroidContext, content, MauiContext);
-
-			_contentView.PlatformView.LayoutParameters = new CoordinatorLayout.LayoutParams(LP.MatchParent, LP.MatchParent)
-			{
-				Behavior = new AppBarLayout.ScrollingViewBehavior()
-			};
-
+			_contentView = new ShellViewRenderer(context, content, MauiContext);
+			ShellRecyclerView.SetLayoutParameters(_contentView.PlatformView, scrollingEnabled: true);
 			return _contentView.PlatformView;
 		}
 
@@ -470,9 +458,9 @@ void OnFlyoutViewLayoutChanging()
 
 		void UpdateVerticalScrollMode()
 		{
-			if (_flyoutContentView is RecyclerView rv && rv.GetLayoutManager() is ScrollLayoutManager lm)
+			if (_flyoutContentView is ShellRecyclerView rv)
 			{
-				lm.ScrollVertically = _shellContext.Shell.FlyoutVerticalScrollMode;
+				ShellRecyclerView.SetLayoutParameters(rv, _shellContext.Shell.FlyoutVerticalScrollMode != ScrollMode.Disabled);
 			}
 		}
 
@@ -796,66 +784,4 @@ void UpdateMinimumHeight()
 			}
 		}
 	}
-
-	class RecyclerViewContainer : RecyclerView
-	{
-		bool _disposed;
-		ScrollLayoutManager _layoutManager;
-
-		public RecyclerViewContainer(Context context) : base(context)
-		{
-			SetClipToPadding(false);
-			SetLayoutManager(_layoutManager = new ScrollLayoutManager(context, (int)Orientation.Vertical, false));
-			SetLayoutManager(new LinearLayoutManager(context, (int)Orientation.Vertical, false));
-		}
-
-		protected override void Dispose(bool disposing)
-		{
-			if (_disposed)
-				return;
-
-			_disposed = true;
-			if (disposing)
-			{
-				SetLayoutManager(null);
-				var adapter = this.GetAdapter();
-				SetAdapter(null);
-				adapter?.Dispose();
-				_layoutManager?.Dispose();
-				_layoutManager = null;
-			}
-
-			base.Dispose(disposing);
-		}
-	}
-
-	internal class ScrollLayoutManager : LinearLayoutManager
-	{
-		public ScrollMode ScrollVertically { get; set; } = ScrollMode.Auto;
-
-		public ScrollLayoutManager(Context context, int orientation, bool reverseLayout) : base(context, orientation, reverseLayout)
-		{
-		}
-
-		int GetVisibleChildCount()
-		{
-			var firstVisibleIndex = FindFirstCompletelyVisibleItemPosition();
-			var lastVisibleIndex = FindLastCompletelyVisibleItemPosition();
-			return lastVisibleIndex - firstVisibleIndex + 1;
-		}
-
-		public override bool CanScrollVertically()
-		{
-			switch (ScrollVertically)
-			{
-				case ScrollMode.Disabled:
-					return false;
-				case ScrollMode.Enabled:
-					return true;
-				default:
-				case ScrollMode.Auto:
-					return ChildCount > GetVisibleChildCount();
-			}
-		}
-	}
 }

src/Controls/tests/DeviceTests/Elements/Shell/ShellTests.Android.cs

@@ -451,18 +451,18 @@ ShellFlyoutRenderer GetDrawerLayout(ShellRenderer shellRenderer)
 			return (ShellFlyoutRenderer)shellContext.CurrentDrawerLayout;
 		}
 
-		RecyclerViewContainer GetFlyoutMenuReyclerView(ShellRenderer shellRenderer)
+		ShellRecyclerView GetFlyoutMenuReyclerView(ShellRenderer shellRenderer)
 		{
 			IShellContext shellContext = shellRenderer;
 			DrawerLayout dl = shellContext.CurrentDrawerLayout;
 
 			var flyout = dl.GetChildAt(0);
-			RecyclerViewContainer flyoutContainer = null;
+			ShellRecyclerView flyoutContainer = null;
 
 			if (dl.GetChildAt(1) is ViewGroup vg1 &&
-				vg1.GetChildAt(0) is RecyclerViewContainer rvc)
+				vg1.GetChildAt(0) is ShellRecyclerView rv)
 			{
-				flyoutContainer = rvc;
+				flyoutContainer = rv;
 			}
 
 			return flyoutContainer ?? throw new Exception("RecyclerView not found");

src/Core/AndroidNative/maui/src/main/java/com/microsoft/maui/ShellRecyclerView.java

@@ -0,0 +1,40 @@
+package com.microsoft.maui;
+
+import android.content.Context;
+import android.view.View;
+
+import androidx.annotation.NonNull;
+import androidx.coordinatorlayout.widget.CoordinatorLayout;
+import androidx.recyclerview.widget.LinearLayoutManager;
+import androidx.recyclerview.widget.RecyclerView;
+
+import com.google.android.material.appbar.AppBarLayout;
+
+/**
+ * Used by Shell flyout menu
+ */
+public class ShellRecyclerView extends RecyclerView {
+    public ShellRecyclerView(@NonNull Context context, Adapter adapter)
+    {
+        super(context);
+
+        setClipToPadding(false);
+        setLayoutManager(new LinearLayoutManager(context, RecyclerView.VERTICAL, false));
+        setLayoutParameters(this, true);
+        setAdapter(adapter);
+    }
+
+    /**
+     * Configures a default, scrollable CoordinatorLayout.LayoutParams that matches its parent
+     * @param view
+     * @param scrollingEnabled
+     */
+    public static void setLayoutParameters(View view, boolean scrollingEnabled)
+    {
+        CoordinatorLayout.LayoutParams layoutParams = new CoordinatorLayout.LayoutParams(CoordinatorLayout.LayoutParams.MATCH_PARENT, CoordinatorLayout.LayoutParams.MATCH_PARENT);
+        if (scrollingEnabled) {
+            layoutParams.setBehavior(new AppBarLayout.ScrollingViewBehavior());
+        }
+        view.setLayoutParams(layoutParams);
+    }
+}