Make XY focus navigation less broken, when there is no starting focused control

src/Avalonia.Base/Input/KeyboardNavigationHandler.cs

@@ -50,34 +50,49 @@ namespace Avalonia.Input
             IInputElement element,
             NavigationDirection direction)
         {
-            return GetNextPrivate(element, direction, null);
+            element = element ?? throw new ArgumentNullException(nameof(element));
+            return GetNextPrivate(element, null, direction, null);
         }
 
         private static IInputElement? GetNextPrivate(
-            IInputElement element,
+            IInputElement? element,
+            IInputRoot? owner,
             NavigationDirection direction,
             KeyDeviceType? keyDeviceType)
         {
-            element = element ?? throw new ArgumentNullException(nameof(element));
+            var elementOrOwner = element ?? owner ?? throw new ArgumentNullException(nameof(owner));
 
             // If there's a custom keyboard navigation handler as an ancestor, use that.
             var custom = (element as Visual)?.FindAncestorOfType<ICustomKeyboardNavigation>(true);
-            if (custom is not null && HandlePreCustomNavigation(custom, element, direction, out var ce))
+            if (custom is not null && HandlePreCustomNavigation(custom, elementOrOwner, direction, out var ce))
                 return ce;
 
-            var result = direction switch
+            IInputElement? result;
+            if (direction is NavigationDirection.Next)
+            {
+                result = TabNavigation.GetNextTab(elementOrOwner, false);
+            }
+            else if (direction is NavigationDirection.Previous)
+            {
+                result = TabNavigation.GetPrevTab(elementOrOwner, null, false);
+            }
+            else if (direction is NavigationDirection.Up or NavigationDirection.Down
+                     or NavigationDirection.Left or NavigationDirection.Right)
+            {
+                // HACK: a window should always have some element focused,
+                // it seems to be a difference between UWP and Avalonia focus manager implementations.
+                result = element is null
+                    ? TabNavigation.GetNextTab(elementOrOwner, true)
+                    : XYFocus.TryDirectionalFocus(direction, element, owner, null, keyDeviceType);
+            }
+            else
             {
-                NavigationDirection.Next => TabNavigation.GetNextTab(element, false),
-                NavigationDirection.Previous => TabNavigation.GetPrevTab(element, null, false),
-                NavigationDirection.Up or NavigationDirection.Down
-                    or NavigationDirection.Left or NavigationDirection.Right
-                    => XYFocus.TryDirectionalFocus(direction, element, null, keyDeviceType),
-                _ => throw new NotSupportedException(),
-            };
+                throw new ArgumentOutOfRangeException(nameof(direction), direction, null);
+            }
 
             // If there wasn't a custom navigation handler as an ancestor of the current element,
             // but there is one as an ancestor of the new element, use that.
-            if (custom is null && HandlePostCustomNavigation(element, result, direction, out ce))
+            if (custom is null && HandlePostCustomNavigation(elementOrOwner, result, direction, out ce))
                 return ce;
 
             return result;
@@ -100,12 +115,7 @@ namespace Avalonia.Input
         // TODO12: remove MovePrivate, and make Move return boolean. Or even remove whole KeyboardNavigationHandler.
         private bool MovePrivate(IInputElement? element, NavigationDirection direction, KeyModifiers keyModifiers, KeyDeviceType? deviceType)
         {
-            if (element is null && _owner is null)
-            {
-                return false;
-            }
-
-            var next = GetNextPrivate(element ?? _owner!, direction, deviceType);
+            var next = GetNextPrivate(element, _owner, direction, deviceType);
 
             if (next != null)
             {

src/Avalonia.Base/Input/Navigation/XYFocus.Impl.cs

@@ -52,7 +52,8 @@ public partial class XYFocus
 
     internal static InputElement? TryDirectionalFocus(
         NavigationDirection direction,
-        IInputElement? element,
+        IInputElement element,
+        IInputElement? owner,
         InputElement? engagedControl,
         KeyDeviceType? keyDeviceType)
     {
@@ -98,7 +99,7 @@ public partial class XYFocus
             KeyDeviceType = keyDeviceType,
             FocusedElementBounds = bounds,
             UpdateManifold = true,
-            SearchRoot = inputElement.GetVisualRoot() as InputElement
+            SearchRoot = owner as InputElement ?? inputElement.GetVisualRoot() as InputElement
         });
     }
 

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

@@ -376,4 +376,46 @@ public class KeyboardNavigationTests_XY : ScopedTestBase
         Assert.Equal(current, FocusManager.GetFocusManager(current)!.GetFocusedElement());
         Assert.False(args.Handled);
     }
+
+    [Fact]
+    public void Can_Focus_Child_Of_Current_Focused()
+    {
+        using var _ = UnitTestApplication.Start(TestServices.FocusableWindow);
+
+        var candidate = new Button() { Height = 20, Width = 20 };
+        var window = new Window
+        {
+            [XYFocus.NavigationModesProperty] = XYFocusNavigationModes.Enabled,
+            Content = candidate,
+            Height = 30
+        };
+        window.Show();
+
+        Assert.Null(KeyboardNavigationHandler.GetNext(window, NavigationDirection.Down));
+    }
+
+    [Fact]
+    public void Can_Focus_Any_Element_If_Nothing_Was_Focused()
+    {
+        // In the future we might auto-focus any element, but for now XY algorithm should be aware of Avalonia specifics.
+        using var _ = UnitTestApplication.Start(TestServices.FocusableWindow);
+
+        var candidate = new Button();
+        var window = new Window
+        {
+            [XYFocus.NavigationModesProperty] = XYFocusNavigationModes.Enabled,
+            Content = new Canvas
+            {
+                Children = { candidate }
+            }
+        };
+        window.Show();
+
+        Assert.Null(FocusManager.GetFocusManager(window)!.GetFocusedElement());
+
+        var args = new KeyEventArgs { RoutedEvent = InputElement.KeyDownEvent, Key = Key.Down, Source = window };
+        window.RaiseEvent(args);
+
+        Assert.Equal(candidate, FocusManager.GetFocusManager(window)!.GetFocusedElement());
+    }
 }