[controls] fix memory leaks in Page & navigation

Fixes: dotnet#13520
Context: https://github.com/danardelean/MauiTestFinalizer

In the above sample, you can see `Page` objects leaking while using
Shell navigation. I was able to reproduce this in a test with both
Shell & `NavigationPage` such as:

    [Fact(DisplayName = "NavigationPage Does Not Leak")]
    public async Task DoesNotLeak()
    {
        SetupBuilder();
        WeakReference pageReference = null;
        var navPage = new NavigationPage(new ContentPage { Title = "Page 1" });

        await CreateHandlerAndAddToWindow<WindowHandlerStub>(new Window(navPage), async (handler) =>
        {
            var page = new ContentPage { Title = "Page 2" };
            pageReference = new WeakReference(page);
            await navPage.Navigation.PushAsync(page);
            await navPage.Navigation.PopAsync();
        });

        await Task.Yield();
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Assert.NotNull(pageReference);
        Assert.False(pageReference.IsAlive, "Page should not be alive!");
    }

After a lot of investigating, I found even a "basic" `Page` test leaks:

    [Fact("Basic Page Does Not Leak")]
    public async Task BasicPageDoesNotLeak()
    {
        var pageReference = new WeakReference(new ContentPage());
        await CreateHandlerAndAddToWindow<PageHandler>((Page)pageReference.Target, _ => Task.CompletedTask);
        await Task.Yield();
        GC.Collect();
        GC.WaitForPendingFinalizers();
        Assert.False(pageReference.IsAlive, "Page should not be alive!");
    }

To summarize:

* Customer's sample only leaks on iOS/Catalyst
* My Shell/NavigationPage tests leak on Android/iOS/Catalyst
* My "basic page" test leaks on all platforms.

Comparing various GC heap dumps in Visual Studio, I was able to find
the sources of these issues. I'll try to list them.

* Tests
    * `CreateHandlerAndAddToWindow` should unset `Window.Page`
    * `ControlsHandlerTestBase.Windows.cs` needed some `null` checks
* Controls.Core
    * `ModalNavigationManager` needs a `Disconnect()` method to unset
      `_previousPage` and call `NavigationModel.Clear()`
    * `Window.Destroy()` then needs to call
      `ModalNavigationManager.Disconnect()`
    * `Window.OnPageChanged()` should unset `_menuBarTracker.Target`
* Windows
    * `ContentViewHandler.DisconnectHandler` should unset
      `platformView.CrossPlatformMeasure` and
      `platformView.CrossPlatformArrange`.
* Android
    * `ContentViewHandler.DisconnectHandler` should unset
      `platformView.CrossPlatformMeasure` and
      `platformView.CrossPlatformArrange`.
    * `ShellContentFragment.Destroy()` should unset `_page`
    * `NavigationViewFragment.OnDestroy()` should unset `_currentView`
      and `_fragmentContainerView`
    * `ScopedFragment.OnDestroy()` should unset `DetailView`
* iOS/Catalyst
    * `ContentViewHandler.DisconnectHandler` should unset
      `platformView.CrossPlatformMeasure` and
      `platformView.CrossPlatformArrange`.

After these changes, my tests pass! They literally would not pass
without fixing all of the above... Each issue I found while reviewing
GC dumps.

WIP:

* I still need to test on iOS/Catalyst & retest user sample
* I need to document how to get `*.gcdump` files w/ Mono runtime

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

@@ -198,6 +198,7 @@ void Destroy()
 			_viewhandler = null;
 			_shellContent = null;
 			_shellPageContainer = null;
+			_page = null;
 		}
 
 		protected override void Dispose(bool disposing)

src/Controls/src/Core/HandlerImpl/Window/Window.Impl.cs

@@ -596,6 +596,7 @@ void OnPageChanged(Page? oldPage, Page? newPage)
 		{
 			if (oldPage != null)
 			{
+				_menuBarTracker.Target = null;
 				InternalChildren.Remove(oldPage);
 				oldPage.HandlerChanged -= OnPageHandlerChanged;
 				oldPage.HandlerChanging -= OnPageHandlerChanging;

src/Controls/tests/DeviceTests/ControlsHandlerTestBase.Windows.cs

@@ -40,10 +40,10 @@ Task SetupWindowForTests<THandler>(IWindow window, Func<Task> runTests, IMauiCon
 				}
 				finally
 				{
-					window.Handler.DisconnectHandler();
+					window.Handler?.DisconnectHandler();
 					await Task.Delay(10);
 					newWindow?.Close();
-					appStub.Handler.DisconnectHandler();
+					appStub.Handler?.DisconnectHandler();
 				}
 			});
 		}

src/Controls/tests/DeviceTests/Elements/NavigationPage/NavigationPageTests.cs

@@ -287,5 +287,28 @@ TabbedPage CreateTabbedPage(string title) => new TabbedPage()
 				};
 			});
 		}
+
+		[Fact(DisplayName = "NavigationPage Does Not Leak")]
+		public async Task DoesNotLeak()
+		{
+			SetupBuilder();
+			WeakReference pageReference = null;
+			var navPage = new NavigationPage(new ContentPage { Title = "Page 1" });
+
+			await CreateHandlerAndAddToWindow<WindowHandlerStub>(new Window(navPage), async (handler) =>
+			{
+				var page = new ContentPage { Title = "Page 2" };
+				pageReference = new WeakReference(page);
+				await navPage.Navigation.PushAsync(page);
+				await navPage.Navigation.PopAsync();
+			});
+
+			await Task.Yield();
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+
+			Assert.NotNull(pageReference);
+			Assert.False(pageReference.IsAlive, "Page should not be alive!");
+		}
 	}
 }

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

@@ -662,7 +662,7 @@ public async Task NavigatedFiresAfterSwitchingFlyoutItems()
 			});
 		}
 
-#if !IOS
+#if !IOS && !MACCATALYST
 		[Fact]
 		public async Task ChangingToNewMauiContextDoesntCrash()
 		{
@@ -919,6 +919,40 @@ public async Task TitleViewMeasures()
 		}
 #endif
 
+		[Fact(DisplayName = "Pages Do Not Leak")]
+		public async Task PagesDoNotLeak()
+		{
+			SetupBuilder();
+			var shell = await CreateShellAsync(shell =>
+			{
+				shell.CurrentItem = new ContentPage() { Title = "Page 1" };
+			});
+
+			WeakReference pageReference = null;
+
+			await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
+			{
+				await OnLoadedAsync(shell.CurrentPage);
+
+				var page = new ContentPage { Title = "Page 2" };
+				pageReference = new WeakReference(page);
+
+				await shell.Navigation.PushAsync(page);
+				await shell.Navigation.PopAsync();
+			});
+
+			// Two GCs required currently
+			for (int i = 0; i < 2; i++)
+			{
+				await Task.Yield();
+				GC.Collect();
+				GC.WaitForPendingFinalizers();
+			}
+
+			Assert.NotNull(pageReference);
+			Assert.False(pageReference.IsAlive, "Page should not be alive!");
+		}
+
 		protected Task<Shell> CreateShellAsync(Action<Shell> action) =>
 			InvokeOnMainThreadAsync(() =>
 			{

src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs

@@ -52,6 +52,8 @@ public static void MapContent(IContentViewHandler handler, IContentView page)
 		protected override void DisconnectHandler(ContentViewGroup platformView)
 		{
 			// If we're being disconnected from the xplat element, then we should no longer be managing its children
+			platformView.CrossPlatformMeasure = null;
+			platformView.CrossPlatformArrange = null;
 			platformView.RemoveAllViews();
 			base.DisconnectHandler(platformView);
 		}

src/Core/src/Handlers/ContentView/ContentViewHandler.Windows.cs

@@ -49,5 +49,12 @@ public static void MapContent(IContentViewHandler handler, IContentView page)
 		{
 			UpdateContent(handler);
 		}
+
+		protected override void DisconnectHandler(ContentPanel platformView)
+		{
+			platformView.CrossPlatformMeasure = null;
+			platformView.CrossPlatformArrange = null;
+			base.DisconnectHandler(platformView);
+		}
 	}
 }

src/Core/src/Handlers/ContentView/ContentViewHandler.iOS.cs

@@ -53,5 +53,15 @@ public static void MapContent(IContentViewHandler handler, IContentView page)
 		{
 			UpdateContent(handler);
 		}
+
+		protected override void DisconnectHandler(ContentView platformView)
+		{
+			platformView.CrossPlatformMeasure = null;
+			platformView.CrossPlatformArrange = null;
+			// TODO: this specific call fixes leaks in iOS/Catalyst
+			// But it doesn't feel quite right
+			platformView.RemoveFromSuperview();
+			base.DisconnectHandler(platformView);
+		}
 	}
 }

src/Core/src/Platform/Android/Navigation/NavigationViewFragment.cs

@@ -78,6 +78,14 @@ public override void OnResume()
 			base.OnResume();
 		}
 
+		public override void OnDestroy()
+		{
+			_currentView = null;
+			_fragmentContainerView = null;
+
+			base.OnDestroy();
+		}
+
 		public override Animation OnCreateAnimation(int transit, bool enter, int nextAnim)
 		{
 			int id = 0;

src/Core/src/Platform/Android/Navigation/ScopedFragment.cs

@@ -1,4 +1,5 @@
-using Android.OS;
+using System;
+using Android.OS;
 using Android.Views;
 using AndroidX.Fragment.App;
 
@@ -8,7 +9,7 @@ class ScopedFragment : Fragment
 	{
 		readonly IMauiContext _mauiContext;
 
-		public IView DetailView { get; private set; }
+		public IView? DetailView { get; private set; }
 
 		public ScopedFragment(IView detailView, IMauiContext mauiContext)
 		{
@@ -19,7 +20,14 @@ public ScopedFragment(IView detailView, IMauiContext mauiContext)
 		public override View OnCreateView(LayoutInflater inflater, ViewGroup? container, Bundle? savedInstanceState)
 		{
 			var pageMauiContext = _mauiContext.MakeScoped(layoutInflater: inflater, fragmentManager: ChildFragmentManager);
-			return DetailView.ToPlatform(pageMauiContext);
+			var detailView = DetailView ?? throw new InvalidOperationException($"DetailView is null. OnDestroy must have been called!");
+			return detailView.ToPlatform(pageMauiContext);
+		}
+
+		public override void OnDestroy()
+		{
+			DetailView = null;
+			base.OnDestroy();
 		}
 	}
 }

src/Core/src/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -9,6 +9,7 @@ override Microsoft.Maui.Handlers.EditorHandler.PlatformArrange(Microsoft.Maui.Gr
 override Microsoft.Maui.Handlers.RadioButtonHandler.PlatformArrange(Microsoft.Maui.Graphics.Rect frame) -> void
 override Microsoft.Maui.Layouts.FlexBasis.Equals(object? obj) -> bool
 override Microsoft.Maui.Layouts.FlexBasis.GetHashCode() -> int
+override Microsoft.Maui.Platform.NavigationViewFragment.OnDestroy() -> void
 override Microsoft.Maui.SizeRequest.Equals(object? obj) -> bool
 override Microsoft.Maui.SizeRequest.GetHashCode() -> int
 static Microsoft.Maui.FontSize.operator !=(Microsoft.Maui.FontSize left, Microsoft.Maui.FontSize right) -> bool

src/Core/src/PublicAPI/net-ios/PublicAPI.Unshipped.txt

@@ -6,6 +6,7 @@ Microsoft.Maui.Hosting.MauiApp.DisposeAsync() -> System.Threading.Tasks.ValueTas
 Microsoft.Maui.Layouts.FlexBasis.Equals(Microsoft.Maui.Layouts.FlexBasis other) -> bool
 Microsoft.Maui.LifecycleEvents.iOSLifecycle.PerformFetch
 Microsoft.Maui.SizeRequest.Equals(Microsoft.Maui.SizeRequest other) -> bool
+override Microsoft.Maui.Handlers.ContentViewHandler.DisconnectHandler(Microsoft.Maui.Platform.ContentView! platformView) -> void
 override Microsoft.Maui.Handlers.SwipeItemButton.Frame.get -> CoreGraphics.CGRect
 override Microsoft.Maui.Handlers.SwipeItemButton.Frame.set -> void
 override Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.ConnectHandler(UIKit.UIButton! platformView) -> void

src/Core/src/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt

@@ -5,6 +5,7 @@ Microsoft.Maui.Handlers.SwipeItemButton.SwipeItemButton() -> void
 Microsoft.Maui.Hosting.MauiApp.DisposeAsync() -> System.Threading.Tasks.ValueTask
 Microsoft.Maui.Layouts.FlexBasis.Equals(Microsoft.Maui.Layouts.FlexBasis other) -> bool
 Microsoft.Maui.SizeRequest.Equals(Microsoft.Maui.SizeRequest other) -> bool
+override Microsoft.Maui.Handlers.ContentViewHandler.DisconnectHandler(Microsoft.Maui.Platform.ContentView! platformView) -> void
 override Microsoft.Maui.Handlers.SwipeItemButton.Frame.get -> CoreGraphics.CGRect
 override Microsoft.Maui.Handlers.SwipeItemButton.Frame.set -> void
 override Microsoft.Maui.Handlers.SwipeItemMenuItemHandler.ConnectHandler(UIKit.UIButton! platformView) -> void

src/Core/src/PublicAPI/net-windows/PublicAPI.Unshipped.txt

@@ -3,6 +3,7 @@ Microsoft.Maui.Hosting.MauiApp.DisposeAsync() -> System.Threading.Tasks.ValueTas
 Microsoft.Maui.Layouts.FlexBasis.Equals(Microsoft.Maui.Layouts.FlexBasis other) -> bool
 Microsoft.Maui.Platform.MauiWebView.MauiWebView(Microsoft.Maui.Handlers.WebViewHandler! handler) -> void
 Microsoft.Maui.SizeRequest.Equals(Microsoft.Maui.SizeRequest other) -> bool
+override Microsoft.Maui.Handlers.ContentViewHandler.DisconnectHandler(Microsoft.Maui.Platform.ContentPanel! platformView) -> void
 override Microsoft.Maui.Layouts.FlexBasis.Equals(object? obj) -> bool
 override Microsoft.Maui.Layouts.FlexBasis.GetHashCode() -> int
 override Microsoft.Maui.SizeRequest.Equals(object? obj) -> bool