fix: Do not allow reentrancy on UpdateLayout even with native layouting

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

@@ -97,13 +97,39 @@ public void Measure(Size availableSize)
 				return;
 			}
 
+			if (IsVisualTreeRoot)
+			{
+				try
+				{
+					_isLayoutingVisualTreeRoot = true;
+					DoMeasure(availableSize);
+				}
+				finally
+				{
+					_isLayoutingVisualTreeRoot = false;
+				}
+			}
+			else
+			{
+				// If possible we avoid the try/finally which might be costly on some platforms
+				DoMeasure(availableSize);
+			}
+		}
+
+		private void DoMeasure(Size availableSize)
+		{
 			InvalidateArrange();
 
 			MeasureCore(availableSize);
 			LayoutInformation.SetAvailableSize(this, availableSize);
 			_isMeasureValid = true;
 		}
 
+		internal virtual void MeasureCore(Size availableSize)
+		{
+			throw new NotSupportedException("UIElement doesn't implement MeasureCore. Inherit from FrameworkElement, which properly implements MeasureCore.");
+		}
+
 		public void Arrange(Rect finalRect)
 		{
 			if (!(this is FrameworkElement))
@@ -119,27 +145,47 @@ public void Arrange(Rect finalRect)
 				return;
 			}
 
-			if (!_isArrangeValid || finalRect != LayoutSlot)
+			if (_isArrangeValid && finalRect == LayoutSlot)
 			{
-				ShowVisual();
-
-				// We must store the updated slot before natively arranging the element,
-				// so the updated value can be read by indirect code that is being invoked on arrange.
-				// For instance, the EffectiveViewPort computation reads that value to detect slot changes (cf. PropagateEffectiveViewportChange)
-				LayoutInformation.SetLayoutSlot(this, finalRect);
+				return;
+			}
 
-				ArrangeCore(finalRect);
-				_isArrangeValid = true;
+			if (IsVisualTreeRoot)
+			{
+				try
+				{
+					_isLayoutingVisualTreeRoot = true;
+					DoArrange(finalRect);
+				}
+				finally
+				{
+					_isLayoutingVisualTreeRoot = false;
+				}
+			}
+			else
+			{
+				// If possible we avoid the try/finally which might be costly on some platforms
+				DoArrange(finalRect);
 			}
 		}
 
+		private void DoArrange(Rect finalRect)
+		{
+			ShowVisual();
+
+			// We must store the updated slot before natively arranging the element,
+			// so the updated value can be read by indirect code that is being invoked on arrange.
+			// For instance, the EffectiveViewPort computation reads that value to detect slot changes (cf. PropagateEffectiveViewportChange)
+			LayoutInformation.SetLayoutSlot(this, finalRect);
+
+			ArrangeCore(finalRect);
+			_isArrangeValid = true;
+		}
+
 		partial void HideVisual();
 		partial void ShowVisual();
 
-		internal virtual void MeasureCore(Size availableSize)
-		{
-			throw new NotSupportedException("UIElement doesn't implement MeasureCore. Inherit from FrameworkElement, which properly implements MeasureCore.");
-		}
+
 
 		internal virtual void ArrangeCore(Rect finalRect)
 		{

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

@@ -52,6 +52,11 @@ public static void RegisterAsScrollPort(UIElement element)
 		/// </summary>
 		internal bool IsWindowRoot { get; set; }
 
+		/// <summary>
+		/// Is this view the top of the managed visual tree
+		/// </summary>
+		internal bool IsVisualTreeRoot { get; set; }
+
 		private void Initialize()
 		{
 			this.SetValue(KeyboardAcceleratorsProperty, new List<KeyboardAccelerator>(0), DependencyPropertyValuePrecedences.DefaultValue);
@@ -341,13 +346,16 @@ private void OpenContextFlyout(object sender, RightTappedRoutedEventArgs args)
 		internal bool IsRenderingSuspended { get; set; }
 
 		[ThreadStatic]
-		private static bool _isInUpdateLayout;
+		private static bool _isInUpdateLayout; // Currently within the UpdateLayout() method (explicit managed layout request)
+
+		[ThreadStatic]
+		private static bool _isLayoutingVisualTreeRoot; // Currenlty in Measure or Arrange of the element flagged with IsVisualTreeRoot (layout requested by the system)
 
 		private const int MaxLayoutIterations = 250;
 
 		public void UpdateLayout()
 		{
-			if (_isInUpdateLayout)
+			if (_isInUpdateLayout || _isLayoutingVisualTreeRoot)
 			{
 				return;
 			}

src/Uno.UI/UI/Xaml/Window.Skia.cs

@@ -145,6 +145,7 @@ private void InternalSetContent(UIElement content)
 
 				_window = new Grid
 				{
+					IsVisualTreeRoot = true,
 					Children =
 					{
 						_rootBorder,

src/Uno.UI/UI/Xaml/Window.cs

@@ -75,6 +75,7 @@ public UIElement Content
 		/// but also the PopupRoot, the DragRoot and all other internal UI elements.
 		/// On platforms like iOS and Android, we might still have few native controls above this.
 		/// </summary>
+		/// <remarks>This element is flagged with IsVisualTreeRoot.</remarks>
 		internal UIElement RootElement => InternalGetRootElement();
 
 		public Rect Bounds { get; private set; }