fix: LayoutSlotWithMarginsAndAlignments is not correct on WASM due to BorderThickness

Author: MartinZikmund

src/Uno.UI/UI/Xaml/FrameworkElementExtensions.cs

@@ -324,6 +324,48 @@ internal static bool TrySetPadding(this IFrameworkElement frameworkElement, Thic
 			return false;
 		}
 
+		internal static bool TryGetActualBorderThickness(this IFrameworkElement frameworkElement, out Thickness borderThickness)
+		{
+			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_Grid_Available && frameworkElement is Grid g)
+			{
+				borderThickness = g.BorderThickness;
+				return true;
+			}
+
+			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_StackPanel_Available && frameworkElement is StackPanel sp)
+			{
+				borderThickness = sp.BorderThickness;
+				return true;
+			}
+
+			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_RelativePanel_Available && frameworkElement is RelativePanel rp)
+			{
+				borderThickness = rp.BorderThickness;
+				return true;
+			}
+
+			if (__LinkerHints.Is_Microsoft_UI_Xaml_Controls_LayoutPanel_Available && frameworkElement is Microsoft.UI.Xaml.Controls.LayoutPanel lp)
+			{
+				borderThickness = lp.BorderThickness;
+				return true;
+			}
+
+			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_ContentPresenter_Available && frameworkElement is ContentPresenter cp)
+			{
+				borderThickness = cp.BorderThickness;
+				return true;
+			}
+
+			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_Border_Available && frameworkElement is Border b)
+			{
+				borderThickness = b.BorderThickness;
+				return true;
+			}
+
+			borderThickness = default;
+			return false;
+		}
+
 		internal static bool TryGetBorderThickness(this IFrameworkElement frameworkElement, out Thickness borderThickness)
 		{
 			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_Grid_Available && frameworkElement is Grid g)
@@ -368,12 +410,6 @@ internal static bool TryGetBorderThickness(this IFrameworkElement frameworkEleme
 				return true;
 			}
 
-			if (__LinkerHints.Is_Windows_UI_Xaml_Controls_CalendarViewBaseItem_Available && frameworkElement is CalendarViewBaseItem cbi)
-			{
-				borderThickness = cbi.GetItemBorderThickness();
-				return true;
-			}
-
 			borderThickness = default;
 			return false;
 		}

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

@@ -219,7 +219,14 @@ protected internal void SetClasses(string[] cssClasses, int index = -1)
 		/// <param name="clipRect">The Clip rect to set, if any</param>
 		protected internal void ArrangeVisual(Rect rect, Rect? clipRect)
 		{
-			LayoutSlotWithMarginsAndAlignments = rect;
+			// FrameworkElement.ArrangeElement shifts the origin by border thickness to adhere to HTML's behavior,
+			// for LayoutSlotWithMarginsAndAlignments we need to shift it back in the right place for consistency
+			// with other platforms.
+			LayoutSlotWithMarginsAndAlignments =
+				VisualTreeHelper.GetParent(this) is FrameworkElement parent &&
+				parent.TryGetActualBorderThickness(out var borderThickness)
+					? new Rect(rect.X + borderThickness.Left, rect.Y + borderThickness.Top, rect.Width, rect.Height)
+					: rect;
 
 			if (FeatureConfiguration.UIElement.AssignDOMXamlProperties)
 			{