Fix focus being locked when restoring focus to a control thats detached from visual tree. (#22113)

* add failing test for detached focus

* Clear focus if restore element is not in focus scope

* check for focus eligibility when restoring focus

* add failing test for second focus scope stealing focus

* if focus scope is not the current focus scope, don't attempt to set focus

Author: emmauss

src/Avalonia.Base/Input/FocusManager.cs

@@ -80,7 +80,22 @@ public bool Focus(
 
             if (_focusRoot?.GetValue(FocusedElementProperty) is { } restore && restore != Current)
             {
-                return FocusCore(keyboardDevice, restore, method, keyModifiers);
+                if (!CanFocus(restore))
+                {
+                    // Previous effective focus is no longer part of the focus root's visual tree. We clear the focused element
+                    _focusRoot.ClearValue(FocusedElementProperty);
+
+                    if (Current != null && GetFocusScope(Current) != _focusRoot)
+                    {
+                        _focusRoot = null;
+
+                        return false;
+                    }
+                }
+                else
+                {
+                    return FocusCore(keyboardDevice, restore, method, keyModifiers);
+                }
             }
 
             _focusRoot = null;

src/Avalonia.Base/Input/InputElement.cs

@@ -568,8 +568,8 @@ protected override void OnDetachedFromVisualTreeCore(VisualTreeAttachmentEventAr
             if (IsFocused)
             {
                 var root = e.AttachmentPoint ?? e.RootVisual;
-                ((FocusManager?)e.PresentationSource.InputRoot.FocusManager)
-                    ?.ClearFocusOnElementRemoved(this, root);
+                (((FocusManager?)e.PresentationSource.InputRoot.FocusManager) ?? 
+                    FocusManager.GetFocusManager(this))?.ClearFocusOnElementRemoved(this, root);
             }
 
             IsKeyboardFocusWithin = false;

tests/Avalonia.Base.UnitTests/Input/InputElement_Focus.cs

@@ -1,4 +1,7 @@
+using System.Linq;
+using Avalonia.Collections;
 using Avalonia.Controls;
+using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.UnitTests;
 using Xunit;
@@ -1629,6 +1632,114 @@ public void Focus_Should_Return_To_First_Window_When_Second_Is_Closed()
             Assert.Same(first, window1.FocusManager.GetFocusedElement());
         }
 
+        [Fact]
+        public void Focus_Should_Stay_In_Active_Window()
+        {
+            using var app = UnitTestApplication.Start(
+                TestServices.StyledWindow.With(keyboardDevice: () => new KeyboardDevice()));
+            var first = new Button { Name = "FirstButton" };
+            var second = new Button { Name = "SecondButton" };
+
+            var window1 = new Window
+            {
+                Content = first
+            };
+
+            var window2 = new Window
+            {
+                Content = second
+            };
+
+            window1.Show();
+
+            // Focus the first button in the first window
+            first.Focus();
+            Assert.Same(first, KeyboardDevice.Instance?.FocusedElement);
+            Assert.Same(first, window1.FocusManager.GetFocusedElement());
+
+            window2.Show();
+
+            // Focus the second button in the second window
+            second.Focus();
+            Assert.Same(second, KeyboardDevice.Instance?.FocusedElement);
+            Assert.Same(second, window2.FocusManager.GetFocusedElement());
+
+            // Activate the first window again
+            window1.PlatformImpl?.Activated?.Invoke();
+
+            // Focus should have moved back to the first button in the first window
+            Assert.Same(first, KeyboardDevice.Instance?.FocusedElement);
+            Assert.Same(first, window1.FocusManager.GetFocusedElement());
+
+            // Close the second window
+            window2.Close();
+
+            // Focus should still be in the first window
+            Assert.Same(first, KeyboardDevice.Instance?.FocusedElement);
+            Assert.Same(first, window1.FocusManager.GetFocusedElement());
+        }
+
+        [Fact]
+        public void Focus_Should_Not_Be_Restored_To_Detached_Control()
+        {
+            using var app = UnitTestApplication.Start(
+                TestServices.StyledWindow.With(keyboardDevice: () => new KeyboardDevice()));
+            var button = new Button { Name = "Button" };
+            MenuItem topLevelMenu;
+            MenuItem childMenu;
+            var menu = new Menu
+            {
+                Items =
+                    {
+                        (topLevelMenu = new MenuItem
+                        {
+                            Header = "Foo",
+                            Items =
+                            {
+                                (childMenu = new MenuItem { Header = "Bar" })
+                            }
+                        }),
+                    }
+            };
+            var panel = new StackPanel();
+            panel.Children.Add(button);
+            panel.Children.Add(menu);
+
+            var window = new Window
+            {
+                Content = panel,
+                Name = "Window1"
+            };
+
+            window.Show();
+
+            // Focus the button
+            button.Focus();
+            Assert.Same(button, KeyboardDevice.Instance?.FocusedElement);
+            Assert.Same(button, window.FocusManager.GetFocusedElement());
+
+
+            // Open the menu and focus the child menu
+            menu.Open();
+            topLevelMenu.IsSubMenuOpen = true;
+            childMenu.Focus();
+
+            // Remove the previously focused button.
+            panel.Children.Remove(button);
+
+
+            // Close the menus.
+            menu.Close();
+            topLevelMenu.Close();
+
+            window.PlatformImpl?.Activated?.Invoke();
+
+            // When window is activated, focus should be empty
+            Assert.Same(null, KeyboardDevice.Instance?.FocusedElement);
+            Assert.Same(null, window.FocusManager.GetFocusedElement());
+        }
+
+
         private class TestFocusScope : Panel, IFocusScope
         {
         }

tests/Avalonia.UnitTests/MockWindowingPlatform.cs

@@ -44,6 +44,7 @@ public static Mock<IWindowImpl> CreateWindowMock(double initialWidth = 800, doub
 
             windowImpl.Setup(x => x.Dispose()).Callback(() =>
             {
+                windowImpl.Object.LostFocus?.Invoke();
                 windowImpl.Object.Closed?.Invoke();
             });