Override OnContentChanged for DropShadowPanel and wire unwire into content's size changed there.

Author: hermitdave

Microsoft.Toolkit.Uwp.UI.Controls/DropShadowPanel/DropShadowPanel.cs

@@ -70,16 +70,34 @@ protected override void OnApplyTemplate()
                 ElementCompositionPreview.SetElementChildVisual(_border, _shadowVisual);
             }
 
-            var content = this.Content as FrameworkElement;
+            ConfigureShadowVisualForCastingElement();
+
+            base.OnApplyTemplate();
+        }
 
-            if (content != null)
+        protected override void OnContentChanged(object oldContent, object newContent)
+        {
+            if (oldContent != null)
             {
-                content.SizeChanged += OnSizeChanged;
+                var oldElement = oldContent as FrameworkElement;
+
+                if (oldElement != null)
+                {
+                    oldElement.SizeChanged -= OnSizeChanged;
+                }
             }
 
-            ConfigureShadowVisualForCastingElement();
+            if (newContent != null)
+            {
+                var newElement = newContent as FrameworkElement;
 
-            base.OnApplyTemplate();
+                if (newElement != null)
+                {
+                    newElement.SizeChanged += OnSizeChanged;
+                }
+            }
+
+            base.OnContentChanged(oldContent, newContent);
         }
 
         private void OnSizeChanged(object sender, SizeChangedEventArgs e)