feat(dragdrop): Fix the drag and drop hit testing

src/Uno.UI/UI/Xaml/DragDrop/DropUITarget.cs

@@ -20,17 +20,19 @@ namespace Windows.UI.Xaml
 {
 	internal class DropUITarget : ICoreDropOperationTarget
 	{
-		private static readonly GetHitTestability _getDropHitTestability = elt =>
+		private static GetHitTestability? _getDropHitTestability;
+		private static GetHitTestability GetDropHitTestability => _getDropHitTestability ??= (elt =>
 		{
 			var visiblity = elt.GetHitTestVisibility();
 			return visiblity switch
 			{
-				HitTestability.Collapsed => HitTestability.Collapsed,
-				_ when !elt.AllowDrop => HitTestability.Invisible,
-				_ => visiblity
+				HitTestability.Collapsed => (HitTestability.Collapsed, _getDropHitTestability!),
+				// Once we reached an element that AllowDrop, we only validate the hit testability for its children
+				_ when elt.AllowDrop => (visiblity, VisualTreeHelper.DefaultGetTestability),
+				_ => (HitTestability.Invisible, _getDropHitTestability!)
 			};
-		};
-			
+		});
+
 
 		// Note: As drag events are routed (so they may be received by multiple elements), we might not have an entry for each drop targets.
 		//		 We will instead have entry only for leaf (a.k.a. OriginalSource).
@@ -124,7 +126,7 @@ public IAsyncOperation<DataPackageOperation> DropAsync(CoreDragInfo dragInfo)
 		{
 			var target = VisualTreeHelper.HitTest(
 				dragInfo.Position,
-				getTestability: _getDropHitTestability,
+				getTestability: GetDropHitTestability,
 				isStale: elt => elt.IsDragOver(dragInfo.SourceId));
 
 			// First raise the drag leave event on stale branch if any.

src/Uno.UI/UI/Xaml/HitTestability.cs

@@ -1,9 +1,11 @@
-using System;
+#nullable enable
+
+using System;
 using System.Linq;
 
 namespace Windows.UI.Xaml
 {
-	internal delegate HitTestability GetHitTestability(UIElement element);
+	internal delegate (HitTestability elementTestability, GetHitTestability childrenGetHitTestability) GetHitTestability(UIElement element);
 
 	internal enum HitTestability
 	{

src/Uno.UI/UI/Xaml/Media/VisualTreeHelper.cs

@@ -250,7 +250,11 @@ internal static IReadOnlyList<_View> ClearChildren(UIElement view)
 #endif
 		}
 
-		private static readonly GetHitTestability _defaultGetTestability = elt => elt.GetHitTestVisibility();
+		internal static readonly GetHitTestability DefaultGetTestability;
+		static VisualTreeHelper()
+		{
+			DefaultGetTestability = elt => (elt.GetHitTestVisibility(), DefaultGetTestability);
+		}
 
 		internal static (UIElement? element, Branch? stale) HitTest(
 			Point position,
@@ -267,7 +271,7 @@ internal static (UIElement? element, Branch? stale) HitTest(
 #endif
 			if (Window.Current.RootElement is UIElement root)
 			{
-				return SearchDownForTopMostElementAt(position, root, getTestability ?? _defaultGetTestability, isStale);
+				return SearchDownForTopMostElementAt(position, root, getTestability ?? DefaultGetTestability, isStale);
 			}
 
 			return default;
@@ -281,7 +285,8 @@ private static (UIElement? element, Branch? stale) SearchDownForTopMostElementAt
 			Func<IEnumerable<UIElement>, IEnumerable<UIElement>>? childrenFilter = null)
 		{
 			var stale = default(Branch?);
-			var elementHitTestVisibility = getVisibility(element);
+			HitTestability elementHitTestVisibility;
+			(elementHitTestVisibility, getVisibility) = getVisibility(element);
 
 #if TRACE_HIT_TESTING
 			using var _ = SET_TRACE_SUBJECT(element);

src/Uno.UI/UI/Xaml/UIElement.netstd.cs

@@ -46,7 +46,11 @@ public partial class UIElement : DependencyObject
 		internal bool IsLoading { get; private set; }
 #endif
 
-		private protected int Depth { get; private set; } = int.MinValue;
+		/// <summary>
+		/// Gets the element depth in the visual tree.
+		/// ** WARNING** This is set before the FrameworkElement loading event and cleared on unload.
+		/// </summary>
+		internal int Depth { get; private set; } = int.MinValue;
 
 		internal static void LoadingRootElement(UIElement visualTreeRoot)
 			=> visualTreeRoot.OnElementLoading(1);