Added Window.OnClosed and sealed HandleClosed.

src/Avalonia.Controls/TopLevel.cs

@@ -271,7 +271,7 @@ namespace Avalonia.Controls
         {
             (this as IInputRoot).MouseDevice?.TopLevelClosed(this);
             PlatformImpl = null;
-            Closed?.Invoke(this, EventArgs.Empty);
+            OnClosed(EventArgs.Empty);
             Renderer?.Dispose();
             Renderer = null;
         }
@@ -317,6 +317,12 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         protected virtual void OnOpened(EventArgs e) => Opened?.Invoke(this, e);
 
+        /// <summary>
+        /// Raises the <see cref="Closed"/> event.
+        /// </summary>
+        /// <param name="e">The event args.</param>
+        protected virtual void OnClosed(EventArgs e) => Closed?.Invoke(this, e);
+
         /// <summary>
         /// Tries to get a service from an <see cref="IAvaloniaDependencyResolver"/>, logging a
         /// warning if not found.

src/Avalonia.Controls/Window.cs

@@ -557,7 +557,7 @@ namespace Avalonia.Controls
             return result;
         }
 
-        protected override void HandleClosed()
+        protected sealed override void HandleClosed()
         {
             RaiseEvent(new RoutedEventArgs(WindowClosedEvent));
 
@@ -565,7 +565,7 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        protected override void HandleResized(Size clientSize)
+        protected sealed override void HandleResized(Size clientSize)
         {
             if (!AutoSizing)
             {