[windows] fix VisualDiagnosticsOverlay keeping Page's alive (#23489)

Fixes: #23034
Context: https://github.com/user-attachments/files/15817814/MauiApp1.zip

In the above sample app, you can observe a memory leak on Windows
by doing:

    Application.Current.MainPage = new Page1(); // This page lives forever
    Application.Current.MainPage = new Page2();

Reviewing memory snapshots, it appears that the
`VisualDiagnosticsOverlay` class is keeping the `Page1` instance
alive:

    MauiApp1.Page1
    -> Microsoft.Maui.Platform.ContentPanel
        -> Microsoft.Maui.Controls.VisualDiagnostics.VisualDiagnosticsOverlay
            -> Microsoft.Maui.Controls.Window

In this case, the `Window` holds a reference to the
`VisualDiagnosticsOverlay`, and the `Window` is still open.
`VisualDiagnosticsOverlay`'s base class is `WindowOverlay`, which
has a `_platformElement` field holding the `ContentPanel` which
holds the `Page1` instance.

In this case, `_platformElement` is only used for unsubscribing
events, so it feels like we can just change it to a `WeakReference`
to solve the issue.

I was able to write a test for this scenario as well.

I skipped the test on iOS/Catalyst, for now, as I was getting the error:

    MauiContext should have been set on parent.

src/Controls/tests/DeviceTests/Memory/MemoryTests.cs

@@ -435,6 +435,37 @@ await CreateHandlerAndAddToWindow(window, async () =>
 		await AssertionExtensions.WaitForGC(references.ToArray());
 	}
 
+	[Fact("VisualDiagnosticsOverlay Does Not Leak"
+#if IOS || MACCATALYST
+		, Skip = "Fails with 'MauiContext should have been set on parent.'"
+#endif
+	)]
+	public async Task VisualDiagnosticsOverlayDoesNotLeak()
+	{
+		SetupBuilder();
+
+		var window = new WindowStub();
+		var overlay = new VisualDiagnosticsOverlay(window);
+		var references = new List<WeakReference>();
+
+		{
+			await InvokeOnMainThreadAsync(async () =>
+			{
+				var page = new ContentPage();
+				window.Content = page;
+				await CreateHandlerAsync(page);
+				overlay.Initialize();
+				references.Add(new(page));
+				references.Add(new(page.Handler));
+				references.Add(new(page.Handler.PlatformView));
+
+				window.Content = null;
+			});
+		}
+
+		await AssertionExtensions.WaitForGC(references.ToArray());
+	}
+
 #if IOS
 	[Fact]
 	public async Task ResignFirstResponderTouchGestureRecognizer()

src/Core/src/WindowOverlay/WindowOverlay.Windows.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 using Microsoft.Maui.Graphics;
 using Microsoft.Maui.Graphics.Platform;
 using Microsoft.Maui.Graphics.Win2D;
@@ -13,7 +14,7 @@ public partial class WindowOverlay
 		W2DGraphicsView? _graphicsView;
 		Frame? _frame;
 		WindowRootViewContainer? _panel;
-		FrameworkElement? _platformElement;
+		WeakReference<FrameworkElement>? _platformElement;
 
 		/// <inheritdoc/>
 		public virtual bool Initialize()
@@ -24,9 +25,10 @@ public virtual bool Initialize()
 			if (Window?.Content == null)
 				return false;
 
-			_platformElement = Window.Content.ToPlatform();
-			if (_platformElement == null)
+			var platformElement = Window.Content.ToPlatform();
+			if (platformElement is null)
 				return false;
+			_platformElement = new(platformElement);
 
 			var handler = Window.Handler as WindowHandler;
 			if (handler?.PlatformView is not Window _window)
@@ -38,7 +40,7 @@ public virtual bool Initialize()
 
 			// Capture when the frame is navigating.
 			// When it is, we will clear existing adorners.
-			if (_platformElement is Frame frame)
+			if (platformElement is Frame frame)
 			{
 				_frame = frame;
 				_frame.Navigating += FrameNavigating;
@@ -48,8 +50,8 @@ public virtual bool Initialize()
 			if (_graphicsView == null)
 				return false;
 
-			_platformElement.Tapped += ViewTapped;
-			_platformElement.PointerMoved += PointerMoved;
+			platformElement.Tapped += ViewTapped;
+			platformElement.PointerMoved += PointerMoved;
 			_graphicsView.Tapped += ViewTapped;
 			_graphicsView.PointerMoved += PointerMoved;
 
@@ -82,10 +84,10 @@ void DeinitializePlatformDependencies()
 		{
 			if (_frame != null)
 				_frame.Navigating -= FrameNavigating;
-			if (_platformElement != null)
+			if (_platformElement is not null && _platformElement.TryGetTarget(out var platformElement))
 			{
-				_platformElement.Tapped -= ViewTapped;
-				_platformElement.PointerMoved -= PointerMoved;
+				platformElement.Tapped -= ViewTapped;
+				platformElement.PointerMoved -= PointerMoved;
 			}
 			if (_graphicsView != null)
 			{