GradientBrush.GradientStops.Clear() leaks brushes when stops are shared #36743

<VerticalStackLayout Padding="20"
            Spacing="12">
        <Label Text="GradientBrush.GradientStops.Clear() Leak Repro"
               FontSize="18"
                FontAttributes="Bold"/>
        <Label Text="A shared GradientStop keeps the GradientBrush alive after Clear() because the CollectionChanged Reset action leaves e.OldItems null, so the PropertyChanged subscription is never removed."
               FontSize="13"
                TextColor="Gray"/>
        <Button Text="1. Create brushes using shared stops"
                Clicked="OnCreateClicked"
                AutomationId="CreateButton"/>
        <Button Text="2. Clear() GradientStops and release"
                Clicked="OnClearAndReleaseClicked"
                AutomationId="ClearButton"/>
        <Button Text="3. Force GC and check leaks"
                Clicked="OnCheckLeaksClicked"
                AutomationId="CheckButton"/>
        <Label x:Name="StatusLabel"
                AutomationId="StatusLabel"
               FontSize="15"
                TextColor="DarkBlue"/>
        <BoxView x:Name="PreviewBox"
                HeightRequest="60"
                CornerRadius="8"/>
    </VerticalStackLayout>

public partial class MainPage : ContentPage
{
	// Shared GradientStops kept alive externally — simulates a shared resource
	// (e.g. a style resource or static geometry reused across pages).
	readonly GradientStop _sharedStop1 = new GradientStop(Colors.DeepSkyBlue, 0f);
	readonly GradientStop _sharedStop2 = new GradientStop(Colors.MediumVioletRed, 1f);

	const int BrushCount = 50;

	// Weak references so we can observe whether the brushes and boxes are collected.
	List<WeakReference> _brushRefs = new();
	List<WeakReference> _boxRefs = new();

	// Strong references released on step 2.
	List<LinearGradientBrush> _brushes = new();
	List<BoxView> _boxes = new();

	public MainPage()
	{
		InitializeComponent();
		UpdateStatus("Step 1: Press \"Create\" to allocate brushes.");
	}

	void OnCreateClicked(object sender, EventArgs e)
	{
		_brushes.Clear();
		_boxes.Clear();
		_brushRefs.Clear();
		_boxRefs.Clear();

		for (int i = 0; i < BrushCount; i++)
		{
			var brush = new LinearGradientBrush
			{
				StartPoint = new Point(0, 0),
				EndPoint = new Point(1, 1),
			};

			// Add the shared stops — subscribes to their PropertyChanged.
			brush.GradientStops.Add(_sharedStop1);
			brush.GradientStops.Add(_sharedStop2);

			var box = new BoxView { HeightRequest = 4 };
			box.Background = brush;

			_brushes.Add(brush);
			_boxes.Add(box);
			_brushRefs.Add(new WeakReference(brush));
			_boxRefs.Add(new WeakReference(box));
		}

		// Show one of the brushes on screen.
		PreviewBox.Background = _brushes[0];

		UpdateStatus($"Created {BrushCount} brushes with shared stops. Now press \"Clear\".");
	}

	void OnClearAndReleaseClicked(object sender, EventArgs e)
	{
		if (_brushes.Count == 0)
		{
			UpdateStatus("Press \"Create\" first.");
			return;
		}

		foreach (var brush in _brushes)
		{
			// BUG: Clear() fires CollectionChanged with Action=Reset.
			// e.OldItems is null so OnGradientStopCollectionChanged never
			// unsubscribes the shared stops, leaving them holding the brush alive.
			brush.GradientStops.Clear();
		}

		// Release all strong references.
		_brushes.Clear();
		_boxes.Clear();
		PreviewBox.Background = null;

		UpdateStatus($"Cleared GradientStops and released {BrushCount} brushes. Now press \"Check Leaks\".");
	}

	void OnCheckLeaksClicked(object sender, EventArgs e)
	{
		if (_brushRefs.Count == 0)
		{
			UpdateStatus("Press \"Create\" and then \"Clear\" first.");
			return;
		}

		GC.Collect();
		GC.WaitForPendingFinalizers();
		GC.Collect();

		int liveBrushes = _brushRefs.Count(r => r.IsAlive);
		int liveBoxes = _boxRefs.Count(r => r.IsAlive);

		string verdict = liveBrushes > 0
			? $"LEAK CONFIRMED: {liveBrushes}/{BrushCount} brushes still alive (shared stops hold them via PropertyChanged). Expected: 0."
			: $"No leak: 0/{BrushCount} brushes alive after GC.";

		StatusLabel.TextColor = liveBrushes > 0 ? Colors.Red : Colors.Green;
		UpdateStatus(verdict);
	}

	void UpdateStatus(string message)
	{
		StatusLabel.Text = message;
	}
}