[controls] fix memory leaks in Page & navigation

[jonathanpeppers]

Fixes #13520
Fixes #12930

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!");
}

To summarize:

• Customer's sample only leaks on iOS/Catalyst

• My Shell/NavigationPage tests leak on Android/iOS/Catalyst

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

• Controls.Core
  • 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.
  • NavigationViewFragment.OnDestroy() should unset _currentView
    and _fragmentContainerView
• iOS/Catalyst
  • ContentViewHandler.DisconnectHandler should unset
    platformView.CrossPlatformMeasure and
    platformView.CrossPlatformArrange.
  • ContainerViewController should call ClearSubviews() on its
    view in WillMoveToParentViewController()
  • ParentingViewController should forward WillMoveToParentViewController()
    to its child view controllers

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.

Other test changes:

• ControlsHandlerTestBase.Windows.cs needed some null checks
• ShellTests needed an && !MACCATALYST to build for MacCatalyst
• Added tests proving we can reuse pages still.

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jonathanpeppers]

My new tests don't pass on iOS yet, so going to see about getting *.gcdump files for iOS similar to:

dotnet/android#7875

[AlleSchonWeg]

Hi @jonathanpeppers ,
is it possible/necessary to port your "leak fixes" to Xamarin Forms?

[jonathanpeppers]

@AlleSchonWeg they might not even apply to Xamarin.Forms I have no idea. The codebase is quite different.

[jonathanpeppers]

Ok, it looks like there is one iOS test failure:

RemovingAllPagesDoesntCrash, System.TimeoutException : Arg_TimeoutException

[rmarinho]

Should we use this to call Disconnect also ?

[jonathanpeppers]

This fixed caused a different problem, so I'm trying 9a36b51 now.

[kojini]

I don't see whether this fix will be backported to .NET 7.
Will it be backported to .NET 7? Do you have ETA on it?
(FYI, I'll be posting the same questions regarding some of memory leak issues/PRs as I'm tracking down when the bug fixes might be released)

[Hooterr]

That looks like something that should've been backported to .NET 7.