fix: fix popups attached to UIElements's position when inside another Window than the MainWindow

Author: Maillard-Sylvain

src/Runtime/Runtime/Core/Rendering/INTERNAL_PopupsManager.cs

@@ -129,15 +129,16 @@ public static Point CalculatePopupAbsolutePositionBasedOnElementPosition(UIEleme
             }
         }
 
+        /// <summary>
+        /// Returns the coordinates of the UIElement, relative to the Window that contains it.
+        /// </summary>
+        /// <param name="element">The element of which the position will be returned.</param>
+        /// <returns>The position of the element relative to the Window that contains it.</returns>
         public static Point GetUIElementAbsolutePosition(UIElement element)
         {
             if (INTERNAL_VisualTreeManager.IsElementInVisualTree(element))
             {
-                GeneralTransform gt = element.TransformToVisual(null);
-
-                // Note: by passing "null" to "TransformToVisual", we tell the framework to
-                // get the coordinates relative to the Window root.
-
+                GeneralTransform gt = element.TransformToVisual(Window.GetWindow(element));
                 return gt.Transform(new Point(0d, 0d));
             }
             return new Point();

src/Runtime/Runtime/System.Windows.Controls.Primitives/Popup.cs

@@ -447,7 +447,7 @@ private void UpdatePosition()
                     try
                     {
                         targetBounds = target
-                            .TransformToVisual(null)
+                            .TransformToVisual(Window.GetWindow(target))
                             .TransformBounds(new Rect(0, 0, target.ActualWidth, target.ActualHeight));
                     }
                     catch { }

src/Runtime/Runtime/System.Windows/Window.cs

@@ -401,6 +401,14 @@ protected void OnClosing(ClosingEventArgs eventArgs)
         [OpenSilver.NotImplemented]
         public static Window GetWindow(DependencyObject dependencyObject)
         {
+            //TODO: this should in theory throw an InvaliOperationException if the dependencyObject is not valid but I didn't check what made it not valid (it looks like Windows themselves are not valid).
+            //      In Silverlight, doing: Window.GetWindow(new Border()); returns the main window if it was made from the main window, null if from another window.
+
+            UIElement dependencyObjectAsUIElement = dependencyObject as UIElement;
+            if (dependencyObjectAsUIElement != null)
+            {
+                return dependencyObjectAsUIElement.INTERNAL_ParentWindow;
+            }
             return null;
         }