allow window to clear focus manager when it closes, preventing memory leak.

src/Avalonia.Controls/WindowBase.cs

@@ -193,6 +193,12 @@ namespace Avalonia.Controls
             try
             {
                 IsVisible = false;
+                
+                if (this is IFocusScope scope)
+                {
+                    FocusManager.Instance?.RemoveFocusScope(scope);
+                }
+                
                 base.HandleClosed();
             }
             finally

src/Avalonia.Input/ApiCompatBaseline.txt

@@ -3,6 +3,7 @@ MembersMustExist : Member 'public Avalonia.Platform.IPlatformHandle Avalonia.Inp
 MembersMustExist : Member 'public Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Input.Gestures.DoubleTappedEvent' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Input.Gestures.RightTappedEvent' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Input.Gestures.TappedEvent' does not exist in the implementation but it does exist in the contract.
+InterfacesShouldHaveSameMembers : Interface member 'public void Avalonia.Input.IFocusManager.ClearFocusScope(Avalonia.Input.IFocusScope)' is present in the implementation but not in the contract.
 MembersMustExist : Member 'public Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Input.InputElement.DoubleTappedEvent' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Interactivity.RoutedEvent<Avalonia.Interactivity.RoutedEventArgs> Avalonia.Input.InputElement.TappedEvent' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Input.InputElement.add_DoubleTapped(System.EventHandler<Avalonia.Interactivity.RoutedEventArgs>)' does not exist in the implementation but it does exist in the contract.
@@ -10,4 +11,4 @@ MembersMustExist : Member 'public void Avalonia.Input.InputElement.add_Tapped(Sy
 MembersMustExist : Member 'public void Avalonia.Input.InputElement.remove_DoubleTapped(System.EventHandler<Avalonia.Interactivity.RoutedEventArgs>)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Input.InputElement.remove_Tapped(System.EventHandler<Avalonia.Interactivity.RoutedEventArgs>)' does not exist in the implementation but it does exist in the contract.
 TypesMustExist : Type 'Avalonia.Platform.IStandardCursorFactory' does not exist in the implementation but it does exist in the contract.
-Total Issues: 11
+Total Issues: 12

src/Avalonia.Input/FocusManager.cs

@@ -145,7 +145,7 @@ namespace Avalonia.Input
         /// Notifies the focus manager of a change in focus scope.
         /// </summary>
         /// <param name="scope">The new focus scope.</param>
-        public void SetFocusScope(IFocusScope scope)
+        public void SetFocusScope(IFocusScope? scope)
         {
             scope = scope ?? throw new ArgumentNullException(nameof(scope));
 
@@ -162,6 +162,22 @@ namespace Avalonia.Input
             Focus(e);
         }
 
+        public void RemoveFocusScope(IFocusScope scope)
+        {
+            scope = scope ?? throw new ArgumentNullException(nameof(scope));
+            
+            if (_focusScopes.TryGetValue(scope, out var e))
+            {
+                SetFocusedElement(scope, null);
+                _focusScopes.Remove(scope);
+            }
+
+            if (Scope == scope)
+            {
+                Scope = null;
+            }
+        }
+
         public static bool GetIsFocusScope(IInputElement e) => e is IFocusScope;
 
         /// <summary>

src/Avalonia.Input/IFocusManager.cs

@@ -35,5 +35,13 @@ namespace Avalonia.Input
         /// when it activates, e.g. when a Window is activated.
         /// </remarks>
         void SetFocusScope(IFocusScope scope);
+
+        /// <summary>
+        /// Notifies the focus manager that a focus scope has been removed.
+        /// </summary>
+        /// <param name="scope">The focus scope to be removed.</param>
+        /// This should not be called by client code. It is called by an <see cref="IFocusScope"/>
+        /// when it deactivates or closes, e.g. when a Window is closed.
+        void RemoveFocusScope(IFocusScope scope);
     }
 }