Merge pull request #3816 from Sergio0694/bugfix/storyboard-event-unsubscribe

Added removal for Storyboard completion handler

Author: Sergio0694

Microsoft.Toolkit.Uwp.UI.Animations/Extensions/StoryboardAnimations.cs

@@ -23,7 +23,14 @@ public static Task BeginAsync(this Storyboard storyboard)
         {
             TaskCompletionSource<object?> taskCompletionSource = new TaskCompletionSource<object?>();
 
-            storyboard.Completed += (_, _) => taskCompletionSource.SetResult(null);
+            void OnCompleted(object sender, object e)
+            {
+                ((Storyboard)sender).Completed -= OnCompleted;
+
+                taskCompletionSource.SetResult(null);
+            }
+
+            storyboard.Completed += OnCompleted;
 
             storyboard.Begin();