/// <summary>A panel that stacks all its children on top of each other, letting them overlay each other.</summary>
    public class OverlayStackPanel : Panel
    {
        public OverlayStackPanel()
        {
            Loaded += OnLoaded;
        }

#if UNITY_5_3_OR_NEWER
        private List<UIElement> InternalChildren { get; set; } = null;
#endif

        /// <inheritdoc/>
        protected override Size MeasureOverride(Size availableSize)
        {
            if (InternalChildren is null)
            {
                return new Size(0, 0);
            }

            double maxWidth = 0;
            double maxHeight = 0;

            for (var index = 0; index < InternalChildren.Count; index++)
            {
                var child = InternalChildren[index];
                child?.Measure(availableSize);

                if (child is null)
                {
                    continue;
                }

                var childDesiredSize = child.DesiredSize;
                maxWidth = Math.Max(maxWidth, childDesiredSize.Width);
                maxHeight = Math.Max(maxHeight, childDesiredSize.Height);
            }

            // Panel size matches the largest child's size
            return new Size((float)maxWidth, (float)maxHeight);
        }

        /// <inheritdoc/>
        protected override Size ArrangeOverride(Size finalSize)
        {
            if (InternalChildren is null)
            {
                return finalSize;
            }

            // Arrange each child to occupy the same space (stacked on top of each other)
            for (var index = 0; index < InternalChildren.Count; index++)
            {
                var child = InternalChildren[index];

                // Positioning each child at (0, 0) ensures they are stacked on top of each other
                child?.Arrange(new Rect(0, 0, finalSize.Width, finalSize.Height));
            }

            return finalSize;
        }

        private void OnLoaded(object sender, RoutedEventArgs args)
        {
#if UNITY_5_3_OR_NEWER
	    // Cache children since calling Children in a loop seems to allocate
            InternalChildren = Children.ToList();
#endif
            Loaded -= OnLoaded;
        }
    }

private static IntPtr VisualGetChild(IntPtr cPtr, int index,
        Visual.ChildrenCountBaseCallback countCallback, Visual.GetChildBaseCallback childCallback)

private static IntPtr VisualGetChild(IntPtr cPtr, int index,
            Visual.ChildrenCountBaseCallback countCallback, Visual.GetChildBaseCallback childCallback) // <<-- Allocations seems to happen here at the native-managed border
        {
            try
            {
                Visual visual = (Visual)GetExtendInstance(cPtr);
                if (visual != null)
                {
                    visual.ChildrenCountBase = countCallback; //<<-- Assigned here
                    visual.GetChildBase = childCallback;
                    Visual child = visual.GetVisualChild(index); // <<-- My override code gets called
                    visual.GetChildBase = null;
                    visual.ChildrenCountBase = null;
                    
                    // [...]