feat: Propagate Popup ZIndex to PopupPanel

Author: MartinZikmund

src/Uno.UI/UI/Xaml/Controls/Popup/Popup.WithPopupRoot.cs

@@ -91,6 +91,10 @@ partial void OnIsOpenChangedPartialNative(bool oldIsOpen, bool newIsOpen)
 					XamlRoot = WinUICoreServices.Instance.ContentRootCoordinator.Unsafe_IslandsIncompatible_CoreWindowContentRoot?.XamlRoot;
 				}
 #endif
+				if (this.IsDependencyPropertySet(Canvas.ZIndexProperty))
+				{
+					Canvas.SetZIndex(PopupPanel, Canvas.GetZIndex(this));
+				}
 
 				// It's important for PopupPanel to be visible before the popup is opened so that
 				// child controls can be IsFocusable, which depends on all ancestors (including PopupPanel)

src/Uno.UI/UI/Xaml/Controls/TextBox/TextBox.skia.cs

@@ -1429,6 +1429,8 @@ private sealed class TextBoxCommand(Action action) : ICommand
 
 	private class CaretWithStemAndThumb : Grid
 	{
+		private const int DefaultZIndex = 10000; // Very high ZIndex to keep carets on top of other popups.
+
 		// This is equal to the default system accent color on Windows.
 		// This is, however, a constant color that doesn't depend on the
 		// current system accent color. Changing the accent color does NOT
@@ -1498,6 +1500,8 @@ public void ShowAt(Point p, XamlRoot xamlRoot)
 				XamlRoot = xamlRoot
 			};
 
+			Canvas.SetZIndex(_popup, DefaultZIndex);
+
 			_popup.HorizontalOffset = p.X;
 			_popup.VerticalOffset = p.Y;
 			if (!_popup.IsOpen)