chore: Improve hit testing tracing

src/Uno.UI/Extensions/UIElementExtensions.cs

@@ -31,34 +31,120 @@ namespace Uno.UI.Extensions
 
 		internal enum IndentationFormat
 		{
+			/// <summary>
+			/// Use tabs to indent log, e.g.
+			/// ```
+			/// [VisualRoot]
+			/// \t[Grid]
+			/// \t\t[TextBlock]
+			/// ```
+			/// </summary>
 			Tabs,
+
+			/// <summary>
+			/// Use tabs to indent log, e.g.
+			/// ```
+			/// [VisualRoot]
+			///   [Grid]
+			///     [TextBlock]
+			/// ```
+			/// </summary>
+			Spaces,
+
+			/// <summary>
+			/// Draw ASCII columns to indent log, e.g.
+			/// ```
+			/// [VisualRoot]
+			/// | [Grid]
+			/// | | [TextBlock]
+			/// ```
+			/// </summary>
 			Columns,
+
+			/// <summary>
+			/// Use the depth numbers of each level to indent log, e.g.
+			/// ```
+			/// [VisualRoot]
+			/// -1> [Grid]
+			/// -1>-2> [TextBlock]
+			/// ```
+			/// </summary>
 			Numbered,
+
+			/// <summary>
+			/// Draw ASCII columns with depth number to indent log, e.g.
+			/// ```
+			/// 01> [VisualRoot]
+			///     | 02> [Grid]
+			///     |     | 03> [TextBlock]
+			/// ```
+			/// </summary>
 			NumberedColumns,
+
+			/// <summary>
+			/// Use spaces with only the current level number to indent the log, e.g.
+			/// ```
+			/// 01> [VisualRoot]
+			///   02> [Grid]
+			///     03> [TextBlock]
+			/// ```
+			/// </summary>
 			NumberedSpaces,
 		}
 
 		private static readonly IndentationFormat _defaultIndentationFormat = IndentationFormat.NumberedSpaces;
 
+		/// <summary>
+		/// The name (<see cref="GetDebugName"/>) indented (<see cref="GetDebugIndent(object?)"/>) for log usage.
+		/// </summary>
 		internal static string GetDebugIdentifier(this object? elt)
+			=> GetDebugIdentifier(elt, _defaultIndentationFormat);
+
+		/// <summary>
+		/// The name (<see cref="GetDebugName"/>) indented (<see cref="GetDebugIndent(object?)"/>) for log usage.
+		/// </summary>
+		internal static string GetDebugIdentifier(this object? elt, IndentationFormat format)
+			=> $"{elt.GetDebugIndent(format)} [{elt.GetDebugName()}]";
+
+		/// <summary>
+		/// Indentation log prefix built from the depth (<see cref="GetDebugDepth"/>) of the element for tree-view like logs.
+		/// </summary>
+		/// <param name="subLine">
+		/// Indicates that the indent is for a sub-line log of the given element,
+		/// so it has to be indented one extra level but without numbering it.
+		/// </param>
+		internal static string GetDebugIndent(this object? elt, bool subLine = false)
+			=> GetDebugIndent(elt, _defaultIndentationFormat, subLine);
+
+		/// <summary>
+		/// Indentation log prefix built from the depth (<see cref="GetDebugDepth"/>) of the element for tree-view like logs.
+		/// </summary>
+		/// <param name="subLine">
+		/// Indicates that the indent is for a sub-line log of the given element,
+		/// so it has to be indented one extra level but without numbering it.
+		/// </param>
+		internal static string GetDebugIndent(this object? elt, IndentationFormat format, bool subLine = false)
 		{
 			var depth = elt.GetDebugDepth();
-			var indentation = _defaultIndentationFormat switch
+			return format switch
 			{
 				IndentationFormat.Numbered when depth < 0 => "-?>",
 				IndentationFormat.NumberedColumns when depth < 0 => "?>",
 				_ when depth < 0 => "",
 
 				IndentationFormat.Columns => GetColumnsIndentation(depth),
-				IndentationFormat.Numbered => GetNumberedIndentation(depth),
-				IndentationFormat.NumberedColumns => GetNumberedColumnIndentation(depth),
-				IndentationFormat.NumberedSpaces => $"{new string(' ', depth * 2)} {depth:D2}>",
-				IndentationFormat.Tabs => $"{new string('\t', depth)}",
-				_ => $"{new string(' ', depth * 2)} {depth:D2}>", // default to NumberedSpaces
+				IndentationFormat.Numbered => GetNumberedIndentation(depth, subLine),
+				IndentationFormat.NumberedColumns => GetNumberedColumnIndentation(depth, subLine),
+				IndentationFormat.Tabs when subLine => new string('\t', depth + 1),
+				IndentationFormat.Tabs => new string('\t', depth),
+				IndentationFormat.Spaces when subLine => new string(' ', (depth + 1) * 2),
+				IndentationFormat.Spaces => new string(' ', depth * 2),
+
+				// default to NumberedSpaces
+				_ when subLine => new string(' ', (depth + 1) * 2 + 1),
+				_ => $"{new string(' ', depth * 2)} {depth:D2}>",
 			};
 
-			return indentation + $"[{elt.GetDebugName()}]";
-
 			static string GetColumnsIndentation(int depth)
 			{
 				var sb = new StringBuilder(depth * 2);
@@ -71,7 +157,7 @@ namespace Uno.UI.Extensions
 				return sb.ToString();
 			}
 
-			static string GetNumberedIndentation(int depth)
+			static string GetNumberedIndentation(int depth, bool subLine)
 			{
 				var sb = new StringBuilder(depth * 4);
 				for (var i = 0; i < depth; i++)
@@ -81,25 +167,37 @@ namespace Uno.UI.Extensions
 					sb.Append('>');
 				}
 
+				if (subLine)
+				{
+					sb.Append(' ', depth.ToString(CultureInfo.InvariantCulture).Length + 2);
+				}
+
 				return sb.ToString();
 			}
 
-			static string GetNumberedColumnIndentation(int depth)
+			static string GetNumberedColumnIndentation(int depth, bool subLine)
 			{
 				var sb = new StringBuilder(depth * 4);
-				for (var i = 0; i < depth - 1; i++)
+				var count = subLine ? depth : depth - 1;
+				for (var i = 0; i < count; i++)
 				{
 					sb.Append(' ', i.ToString(CultureInfo.InvariantCulture).Length);
 					sb.Append('|');
 				}
 
-				sb.Append(depth);
-				sb.Append('>');
+				if (!subLine)
+				{
+					sb.Append(depth);
+					sb.Append('>');
+				}
 
 				return sb.ToString();
 			}
 		}
 
+		/// <summary>
+		/// Gets the depth of an element in the visual tree.
+		/// </summary>
 		internal static int GetDebugDepth(this object? elt) =>
 			elt switch
 			{

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

@@ -131,7 +131,7 @@ namespace Windows.UI.Xaml
 				dragInfo.Position,
 				Window.Current.RootElement.XamlRoot, //TODO: Choose proper XamlRoot https://github.com/unoplatform/uno/issues/8978
 				getTestability: GetDropHitTestability,
-				isStale: elt => elt.IsDragOver(dragInfo.SourceId));
+				isStale: new (elt => elt.IsDragOver(dragInfo.SourceId), "IsDragOver"));
 
 			// First raise the drag leave event on stale branch if any.
 			if (target.stale is { } staleBranch)

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

@@ -30,4 +30,6 @@ namespace Windows.UI.Xaml
 		/// </summary>
 		Visible,
 	}
+
+	internal record struct StalePredicate(Predicate<UIElement> Method, string Name);
 }

src/Uno.UI/UI/Xaml/Internal/InputManager.Pointers.Managed.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using Uno.Foundation.Extensibility;
 using Uno.Foundation.Logging;
 using Uno.UI.Extensions;
@@ -60,7 +61,7 @@ internal partial class InputManager
 		private static IPointerExtension? _pointerExtension;
 
 		// TODO: Use pointer ID for the predicates
-		private static readonly Predicate<UIElement> _isOver = e => e.IsPointerOver;
+		private static readonly StalePredicate _isOver = new(e => e.IsPointerOver, "IsPointerOver");
 
 		private readonly Dictionary<Pointer, UIElement> _pressedElements = new();
 
@@ -107,7 +108,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerWheel [{originalSource.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerWheelChanged [{originalSource.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -116,16 +117,6 @@ internal partial class InputManager
 			RaiseUsingCaptures(Wheel, originalSource, routedArgs);
 		}
 
-		private (UIElement?, VisualTreeHelper.Branch?) HitTest(PointerEventArgs args, Predicate<UIElement>? isStale = null)
-		{
-			if (_inputManager._contentRoot?.XamlRoot is null)
-			{
-				throw new InvalidOperationException("The XamlRoot must be properly initialized for hit testng.");
-			}
-
-			return VisualTreeHelper.HitTest(args.CurrentPoint.Position, _inputManager._contentRoot.XamlRoot, isStale: isStale);
-		}
-
 		internal void OnPointerEntered(Windows.UI.Core.PointerEventArgs args)
 		{
 			var (originalSource, _) = HitTest(args);
@@ -151,7 +142,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerEntered [{originalSource.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerEntered [{originalSource.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -163,7 +154,7 @@ internal partial class InputManager
 		{
 			// This is how UWP behaves: when out of the bounds of the Window, the root element is use.
 			var originalSource = Windows.UI.Xaml.Window.Current.Content;
-			if (originalSource == null)
+			if (originalSource is null)
 			{
 				if (this.Log().IsEnabled(LogLevel.Trace))
 				{
@@ -174,7 +165,6 @@ internal partial class InputManager
 			}
 
 			var overBranchLeaf = VisualTreeHelper.SearchDownForLeaf(originalSource, _isOver);
-
 			if (overBranchLeaf is null)
 			{
 				if (this.Log().IsEnabled(LogLevel.Trace))
@@ -187,7 +177,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerExited [{overBranchLeaf.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerExited [{overBranchLeaf.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -222,7 +212,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerPressed [{originalSource.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerPressed [{originalSource.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -254,7 +244,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerReleased [{originalSource.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerReleased [{originalSource.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -289,7 +279,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerMoved [{originalSource.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerMoved [{originalSource.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -297,16 +287,22 @@ internal partial class InputManager
 			// First raise the PointerExited events on the stale branch
 			if (staleBranch.HasValue)
 			{
-				Raise(Leave, staleBranch.Value, routedArgs);
+				if (Raise(Leave, staleBranch.Value, routedArgs) is { VisualTreeAltered: true })
+				{
+					// The visual tree have been modified in a way that requires to perform a new hit test.
+					originalSource = HitTest(args).element ?? Windows.UI.Xaml.Window.Current.Content;
+				}
 			}
 
 			// Second (try to) raise the PointerEnter on the OriginalSource
 			// Note: This won't do anything if already over.
-			routedArgs.Handled = false;
-			Raise(Enter, originalSource, routedArgs);
+			if (Raise(Enter, originalSource, routedArgs) is { VisualTreeAltered: true })
+			{
+				// The visual tree have been modified in a way that requires to perform a new hit test.
+				originalSource = HitTest(args).element ?? Windows.UI.Xaml.Window.Current.Content;
+			}
 
 			// Finally raise the event, either on the OriginalSource or on the capture owners if any
-			routedArgs.Handled = false;
 			RaiseUsingCaptures(Move, originalSource, routedArgs);
 		}
 
@@ -330,7 +326,7 @@ internal partial class InputManager
 
 			if (this.Log().IsEnabled(LogLevel.Trace))
 			{
-				this.Log().Trace($"CoreWindow_PointerCancelled [{originalSource.GetDebugName()}");
+				this.Log().Trace($"CoreWindow_PointerCancelled [{originalSource.GetDebugName()}]");
 			}
 
 			var routedArgs = new PointerRoutedEventArgs(args, originalSource);
@@ -344,7 +340,7 @@ internal partial class InputManager
 		{
 			if (_pointerExtension is not null)
 			{
-				_pointerExtension?.SetPointerCapture(uniqueId, _inputManager._contentRoot.XamlRoot);
+				_pointerExtension.SetPointerCapture(uniqueId, _inputManager._contentRoot.XamlRoot);
 			}
 			else
 			{
@@ -356,7 +352,7 @@ internal partial class InputManager
 		{
 			if (_pointerExtension is not null)
 			{
-				_pointerExtension?.ReleasePointerCapture(uniqueId, _inputManager._contentRoot.XamlRoot);
+				_pointerExtension.ReleasePointerCapture(uniqueId, _inputManager._contentRoot.XamlRoot);
 			}
 			else
 			{
@@ -368,7 +364,7 @@ internal partial class InputManager
 		{
 			if (PointerCapture.TryGet(routedArgs.Pointer, out var capture))
 			{
-				foreach (var target in capture.Targets)
+				foreach (var target in capture.Targets.ToList())
 				{
 					target.Element.ReleasePointerCapture(capture.Pointer.UniqueId, kinds: PointerCaptureKind.Any);
 				}
@@ -392,11 +388,22 @@ internal partial class InputManager
 		}
 
 		#region Helpers
+		private (UIElement? element, VisualTreeHelper.Branch? stale) HitTest(PointerEventArgs args, StalePredicate? isStale = null)
+		{
+			if (_inputManager._contentRoot.XamlRoot is null)
+			{
+				throw new InvalidOperationException("The XamlRoot must be properly initialized for hit testing.");
+			}
+
+			return VisualTreeHelper.HitTest(args.CurrentPoint.Position, _inputManager._contentRoot.XamlRoot, isStale: isStale);
+		}
+
 		private delegate void RaisePointerEventArgs(UIElement element, PointerRoutedEventArgs args, BubblingContext ctx);
+		private readonly record struct PointerEvent(RaisePointerEventArgs Invoke, [CallerMemberName] string Name = "");
 
-		private static readonly RaisePointerEventArgs Wheel = (elt, args, ctx) => elt.OnPointerWheel(args, ctx);
-		private static readonly RaisePointerEventArgs Enter = (elt, args, ctx) => elt.OnPointerEnter(args, ctx);
-		private static readonly RaisePointerEventArgs Leave = (elt, args, ctx) =>
+		private static readonly PointerEvent Wheel = new((elt, args, ctx) => elt.OnPointerWheel(args, ctx));
+		private static readonly PointerEvent Enter = new((elt, args, ctx) => elt.OnPointerEnter(args, ctx));
+		private static readonly PointerEvent Leave = new((elt, args, ctx) =>
 		{
 			elt.OnPointerExited(args, ctx);
 
@@ -407,38 +414,70 @@ internal partial class InputManager
 			ctx.IsInternal = true;
 			args.Handled = false;
 			elt.OnPointerUp(args, ctx);
-		};
-		private static readonly RaisePointerEventArgs Pressed = (elt, args, ctx) => elt.OnPointerDown(args, ctx);
-		private static readonly RaisePointerEventArgs Released = (elt, args, ctx) => elt.OnPointerUp(args, ctx);
-		private static readonly RaisePointerEventArgs Move = (elt, args, ctx) => elt.OnPointerMove(args, ctx);
-		private static readonly RaisePointerEventArgs Cancelled = (elt, args, ctx) => elt.OnPointerCancel(args, ctx);
+		});
+		private static readonly PointerEvent Pressed = new((elt, args, ctx) => elt.OnPointerDown(args, ctx));
+		private static readonly PointerEvent Released = new((elt, args, ctx) => elt.OnPointerUp(args, ctx));
+		private static readonly PointerEvent Move = new((elt, args, ctx) => elt.OnPointerMove(args, ctx));
+		private static readonly PointerEvent Cancelled = new((elt, args, ctx) => elt.OnPointerCancel(args, ctx));
+
+		private static PointerEventDispatchResult Raise(PointerEvent evt, UIElement originalSource, PointerRoutedEventArgs routedArgs)
+		{
+			routedArgs.Handled = false;
+			UIElement.UIElement.BeginPointerEventDispatch();
 
-		private static void Raise(RaisePointerEventArgs raise, UIElement originalSource, PointerRoutedEventArgs routedArgs)
-			=> raise(originalSource, routedArgs, BubblingContext.Bubble);
+			evt.Invoke(originalSource, routedArgs, BubblingContext.Bubble);
 
-		private static void Raise(RaisePointerEventArgs raise, VisualTreeHelper.Branch branch, PointerRoutedEventArgs routedArgs)
-			=> raise(branch.Leaf, routedArgs, BubblingContext.BubbleUpTo(branch.Root));
+			return EndPointerEventDispatch();
+		}
+
+		private static PointerEventDispatchResult Raise(PointerEvent evt, VisualTreeHelper.Branch branch, PointerRoutedEventArgs routedArgs)
+		{
+			routedArgs.Handled = false;
+			UIElement.BeginPointerEventDispatch();
+
+			evt.Invoke(branch.Leaf, routedArgs, BubblingContext.BubbleUpTo(branch.Root));
+
+			return UIElement.EndPointerEventDispatch();
+		}
 
-		private static void RaiseUsingCaptures(RaisePointerEventArgs raise, UIElement originalSource, PointerRoutedEventArgs routedArgs)
+		private PointerEventDispatchResult RaiseUsingCaptures(PointerEvent evt, UIElement originalSource, PointerRoutedEventArgs routedArgs)
 		{
+			routedArgs.Handled = false;
+			UIElement.BeginPointerEventDispatch();
+
 			if (PointerCapture.TryGet(routedArgs.Pointer, out var capture))
 			{
 				var targets = capture.Targets.ToList();
 				if (capture.IsImplicitOnly)
 				{
-					raise(originalSource, routedArgs, BubblingContext.Bubble);
+					if (this.Log().IsEnabled(LogLevel.Trace))
+					{
+						this.Log().Trace($"[Implicit capture] raising event {evt.Name} (args: {routedArgs.GetHashCode():X8}) to original source first [{originalSource.GetDebugName()}]");
+					}
+
+					evt.Invoke(originalSource, routedArgs, BubblingContext.Bubble);
 
 					foreach (var target in targets)
 					{
+						if (this.Log().IsEnabled(LogLevel.Trace))
+						{
+							this.Log().Trace($"[Implicit capture] raising event {evt.Name} (args: {routedArgs.GetHashCode():X8}) to capture target [{originalSource.GetDebugName()}] (-- no bubbling--)");
+						}
+
 						routedArgs.Handled = false;
-						raise(target.Element, routedArgs, BubblingContext.NoBubbling);
+						evt.Invoke(target.Element, routedArgs, BubblingContext.NoBubbling);
 					}
 				}
 				else
 				{
 					var explicitTarget = targets.Find(c => c.Kind.HasFlag(PointerCaptureKind.Explicit))!;
 
-					raise(explicitTarget.Element, routedArgs, BubblingContext.Bubble);
+					if (this.Log().IsEnabled(LogLevel.Trace))
+					{
+						this.Log().Trace($"[Explicit capture] raising event {evt.Name} (args: {routedArgs.GetHashCode():X8}) to capture target [{explicitTarget.Element.GetDebugName()}]");
+					}
+
+					evt.Invoke(explicitTarget.Element, routedArgs, BubblingContext.Bubble);
 
 					foreach (var target in targets)
 					{
@@ -447,15 +486,27 @@ internal partial class InputManager
 							continue;
 						}
 
+						if (this.Log().IsEnabled(LogLevel.Trace))
+						{
+							this.Log().Trace($"[Explicit capture] raising event {evt.Name} (args: {routedArgs.GetHashCode():X8}) to alternative (implicit) target [{explicitTarget.Element.GetDebugName()}] (-- no bubbling--)");
+						}
+
 						routedArgs.Handled = false;
-						raise(target.Element, routedArgs, BubblingContext.NoBubbling);
+						evt.Invoke(target.Element, routedArgs, BubblingContext.NoBubbling);
 					}
 				}
 			}
 			else
 			{
-				raise(originalSource, routedArgs, BubblingContext.Bubble);
+				if (this.Log().IsEnabled(LogLevel.Trace))
+				{
+					this.Log().Trace($"[No capture] raising event {evt.Name} (args: {routedArgs.GetHashCode():X8}) to original source [{originalSource.GetDebugName()}]");
+				}
+
+				evt.Invoke(originalSource, routedArgs, BubblingContext.Bubble);
 			}
+
+			return UIElement.EndPointerEventDispatch();
 		}
 		#endregion
 	}

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

 #nullable disable // Not supported by WinUI yet
-// #define TRACE_HIT_TESTING
+//#define TRACE_HIT_TESTING
 
 using System;
 using System.Collections.Generic;
@@ -353,12 +353,12 @@ namespace Windows.UI.Xaml.Media
 			Point position,
 			XamlRoot? xamlRoot,
 			GetHitTestability? getTestability = null,
-			Predicate<UIElement>? isStale = null
+			StalePredicate? isStale = null
 #if TRACE_HIT_TESTING
-			, [CallerMemberName] string? caller = null)
+			, [CallerMemberName] string caller = "")
 		{
 			using var _ = BEGIN_TRACE();
-			TRACE($"[{caller!.Replace("CoreWindow_Pointer", "").ToUpperInvariant()}] @{position.ToDebugString()}");
+			TRACE($"[{caller!.ToUpperInvariant()}] @{position.ToDebugString()}");
 #else
 			)
 		{
@@ -375,7 +375,7 @@ namespace Windows.UI.Xaml.Media
 			Point posRelToParent,
 			UIElement element,
 			GetHitTestability getVisibility,
-			Predicate<UIElement>? isStale = null,
+			StalePredicate? isStale = null,
 			Func<IEnumerable<UIElement>, IEnumerable<UIElement>>? childrenFilter = null)
 		{
 			var stale = default(Branch?);
@@ -391,9 +391,9 @@ namespace Windows.UI.Xaml.Media
 			if (elementHitTestVisibility == HitTestability.Collapsed)
 			{
 				// Even if collapsed, if the element is stale, we search down for the real stale leaf
-				if (isStale?.Invoke(element) ?? false)
+				if (isStale?.Method.Invoke(element) ?? false)
 				{
-					stale = SearchDownForStaleBranch(element, isStale);
+					stale = SearchDownForStaleBranch(element, isStale.Value);
 				}
 
 				TRACE($"> NOT FOUND (Element is HitTestability.Collapsed) | stale branch: {stale?.ToString() ?? "-- none --"}");
@@ -467,9 +467,18 @@ namespace Windows.UI.Xaml.Media
 			if (!clippingBounds.Contains(posRelToElement))
 			{
 				// Even if out of bounds, if the element is stale, we search down for the real stale leaf
-				if (isStale?.Invoke(element) ?? false)
+				if (isStale is not null)
 				{
-					stale = SearchDownForStaleBranch(element, isStale);
+					if (isStale.Value.Method(element))
+					{
+						TRACE($"- Is {isStale.Value.Name}");
+
+						stale = SearchDownForStaleBranch(element, isStale.Value);
+					}
+					else
+					{
+						TRACE($"- Is NOT {isStale.Value.Name}");
+					}
 				}
 
 				TRACE($"> NOT FOUND (Out of the **clipped** bounds) | stale branch: {stale?.ToString() ?? "-- none --"}");
@@ -485,29 +494,65 @@ namespace Windows.UI.Xaml.Media
 				var childResult = SearchDownForTopMostElementAt(posRelToElement, child.Current!, getVisibility, isChildStale);
 
 				// If we found a stale element in child sub-tree, keep it and stop looking for stale elements
-				if (childResult.stale is { })
+				if (childResult.stale is not null)
 				{
 					stale = childResult.stale;
 					isChildStale = null;
 				}
 
 				// If we found an acceptable element in the child's sub-tree, job is done!
-				if (childResult.element is { })
+				if (childResult.element is not null)
 				{
-					if (isChildStale is { }) // Also indicates that stale is null
+					if (isChildStale is not null) // Also indicates that stale is null
 					{
 						// If we didn't find any stale root in previous children or in the child's sub tree,
 						// we continue to enumerate sibling children to detect a potential stale root.
 
+						TRACE($"+ Searching for stale {isChildStale.Value.Name} branch.");
+
 						while (child.MoveNext())
 						{
-							if (isChildStale(child.Current))
+#if TRACE_HIT_TESTING
+							using var __ = SET_TRACE_SUBJECT(child.Current);
+#endif
+
+							if (isChildStale.Value.Method(child.Current))
 							{
-								stale = SearchDownForStaleBranch(child.Current!, isChildStale);
+								TRACE($"- Is {isChildStale.Value.Name}");
+
+								stale = SearchDownForStaleBranch(child.Current!, isChildStale.Value);
+
+#if TRACE_HIT_TESTING
+								while (child.MoveNext())
+								{
+									using var ___ = SET_TRACE_SUBJECT(child.Current);
+									if (isChildStale.Value.Method(child.Current))
+									{
+										//Debug.Assert(false);
+										TRACE($"- Is {isChildStale.Value.Name} ***** INVALID: Only one branch can be considered as stale at once! ****");
+									}
+									TRACE($"> Ignored since leaf and stale branch has already been found.");
+								}
+#endif
+
 								break;
 							}
+							else
+							{
+								TRACE($"- Is NOT {isChildStale.Value.Name}");
+							}
+						}
+					}
+#if TRACE_HIT_TESTING
+					else
+					{
+						while (child.MoveNext())
+						{
+							using var __ = SET_TRACE_SUBJECT(child.Current);
+							TRACE($"> Ignored since leaf has already been found and no stale branch to find.");
 						}
 					}
+#endif
 
 					TRACE($"> found child: {childResult.element.GetDebugName()} | stale branch: {stale?.ToString() ?? "-- none --"}");
 					return (childResult.element, stale);
@@ -525,7 +570,7 @@ namespace Windows.UI.Xaml.Media
 			{
 				// If no stale element found yet, validate if the current is stale.
 				// Note: no needs to search down for stale child, we already did it!
-				if (isStale?.Invoke(element) ?? false)
+				if (isStale?.Method.Invoke(element) ?? false)
 				{
 					stale = new Branch(element, stale?.Leaf ?? element);
 				}
@@ -535,16 +580,33 @@ namespace Windows.UI.Xaml.Media
 			}
 		}
 
-		private static Branch SearchDownForStaleBranch(UIElement staleRoot, Predicate<UIElement> isStale)
-			=> new Branch(staleRoot, SearchDownForLeaf(staleRoot, isStale));
+		private static Branch SearchDownForStaleBranch(UIElement staleRoot, StalePredicate isStale)
+			=> new Branch(staleRoot, SearchDownForLeafCore(staleRoot, isStale));
+
+		internal static UIElement SearchDownForLeaf(UIElement root, StalePredicate predicate)
+		{
+#if TRACE_HIT_TESTING
+			using var trace = ENSURE_TRACE();
+#endif
+			return SearchDownForLeafCore(root, predicate);
+		}
 
-		internal static UIElement SearchDownForLeaf(UIElement root, Predicate<UIElement> predicate)
+		private static UIElement SearchDownForLeafCore(UIElement root, StalePredicate predicate)
 		{
 			foreach (var child in GetManagedVisualChildren(root).Reverse())
 			{
-				if (predicate(child))
+#if TRACE_HIT_TESTING
+				SET_TRACE_SUBJECT(child);
+#endif
+
+				if (predicate.Method(child))
 				{
-					return SearchDownForLeaf(child, predicate);
+					TRACE($"- Is {predicate.Name}");
+					return SearchDownForLeafCore(child, predicate);
+				}
+				else
+				{
+					TRACE($"- Is NOT {predicate.Name}");
 				}
 			}
 
@@ -560,7 +622,7 @@ namespace Windows.UI.Xaml.Media
 			}
 		}
 
-		#region Helpers
+#region Helpers
 		private static Func<IEnumerable<UIElement>, IEnumerable<UIElement>> Except(UIElement element)
 			=> children => children.Except(element);
 
@@ -585,12 +647,17 @@ namespace Windows.UI.Xaml.Media
 			}
 		}
 
+		internal static IEnumerable<UIElement> GetManagedVisualChildren(object view)
+			=> view is _ViewGroup elt
+				? GetManagedVisualChildren(elt)
+				: Enumerable.Empty<UIElement>();
+
 #if __IOS__ || __MACOS__ || __ANDROID__
 		/// <summary>
 		/// Gets all immediate UIElement children of this <paramref name="view"/>. If any immediate subviews are native, it will descend into
 		/// them depth-first until it finds a UIElement, and return those UIElements.
 		/// </summary>
-		private static IEnumerable<UIElement> GetManagedVisualChildren(_ViewGroup view)
+		internal static IEnumerable<UIElement> GetManagedVisualChildren(_ViewGroup view)
 		{
 			foreach (var child in view.GetChildren())
 			{
@@ -608,11 +675,12 @@ namespace Windows.UI.Xaml.Media
 			}
 		}
 #else
-		private static IEnumerable<UIElement> GetManagedVisualChildren(_View view) => view.GetChildren().OfType<UIElement>();
+		internal static IEnumerable<UIElement> GetManagedVisualChildren(_View view)
+			=> view.GetChildren().OfType<UIElement>();
 #endif
-		#endregion
+#endregion
 
-		#region HitTest tracing
+#region HitTest tracing
 #if TRACE_HIT_TESTING
 		[ThreadStatic]
 		private static StringBuilder? _trace;
@@ -631,6 +699,9 @@ namespace Windows.UI.Xaml.Media
 			});
 		}
 
+		private static IDisposable ENSURE_TRACE()
+			=> _trace is null ? BEGIN_TRACE() : Disposable.Empty;
+
 		private static IDisposable SET_TRACE_SUBJECT(UIElement element)
 		{
 			if (_trace is { })
@@ -638,8 +709,7 @@ namespace Windows.UI.Xaml.Media
 				var previous = _traceSubject;
 				_traceSubject = element;
 
-				_trace.Append(new string('\t', _traceSubject.Depth - 1));
-				_trace.Append($"[{element.GetDebugName()}]\r\n");
+				_trace.AppendLine(_traceSubject.GetDebugIdentifier());
 
 				return Disposable.Create(() => _traceSubject = previous);
 			}
@@ -656,7 +726,8 @@ namespace Windows.UI.Xaml.Media
 #if TRACE_HIT_TESTING
 			if (_trace is { })
 			{
-				_trace.Append(new string('\t', _traceSubject?.Depth ?? 0));
+				_trace.Append(_traceSubject.GetDebugIndent(subLine: true));
+				_trace.Append(' ');
 				_trace.Append(msg.ToStringInvariant());
 				_trace.Append("\r\n");
 			}

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

@@ -646,7 +646,7 @@ namespace Windows.UI.Xaml
 		internal bool RaiseEvent(RoutedEvent routedEvent, RoutedEventArgs args, BubblingContext ctx = default)
 		{
 #if TRACE_ROUTED_EVENT_BUBBLING
-			Debug.Write($"{this.GetDebugIdentifier()} - [{routedEvent.Name.TrimEnd("Event")}] (ctx: {ctx})\r\n");
+			global::System.Diagnostics.Debug.Write($"{this.GetDebugIdentifier()} - [{routedEvent.Name.TrimEnd("Event")}-{args?.GetHashCode():X8}] (ctx: {ctx}){(this is Windows.UI.Xaml.Controls.ContentControl ctrl ? ctrl.DataContext : "")}\r\n");
 #endif
 
 			if (routedEvent.Flag == RoutedEventFlag.None)
@@ -845,12 +845,8 @@ namespace Windows.UI.Xaml
 			/// </remarks>
 			public bool IsInternal { get; set; }
 
-			public BubblingContext WithMode(BubblingMode mode) => new()
-			{
-				Mode = mode,
-				Root = Root,
-				IsInternal = IsInternal
-			};
+			public BubblingContext WithMode(BubblingMode mode)
+				=> this with { Mode = mode };
 
 			public override string ToString()
 				=> $"{Mode}{(IsInternal ? " *internal*" : "")}{(Root is { } r ? $" up to {Root.GetDebugName()}" : "")}";