UWP/WinUI style XYFocus subtree restrictions (#16557)

* Basic failing unit test for UWP/WinUI XYFocus search boundary scenario.

* Change IsAllowedXYNavigationMode to return false if keyDeviceType is null and modes == disabled.

* Add helper function to find the closest InputElement to the target element whose parent does not allow XYFocus rather than always searching from the TopLevel.  This restricts focus searches within a specific subtree rather than allowing searches to bridge subtrees that share an XYFocus disabled parent.

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

@@ -92,6 +92,11 @@ public partial class XYFocus
             return null;
         }
 
+        if(!(XYFocusHelpers.FindXYSearchRoot(inputElement, keyDeviceType) is InputElement searchRoot))
+        {
+            return null;
+        }
+
         _instance.SetManifoldsFromBounds(bounds);
 
         return _instance.GetNextFocusableElement(direction, inputElement, engagedControl, true, new XYFocusOptions
@@ -99,7 +104,7 @@ public partial class XYFocus
             KeyDeviceType = keyDeviceType,
             FocusedElementBounds = bounds,
             UpdateManifold = true,
-            SearchRoot = owner as InputElement ?? inputElement.GetVisualRoot() as InputElement
+            SearchRoot = searchRoot
         });
     }
 

src/Avalonia.Base/Input/Navigation/XYFocusHelpers.cs

@@ -1,4 +1,5 @@
 using System;
+using Avalonia.VisualTree;
 
 namespace Avalonia.Input;
 
@@ -13,11 +14,25 @@ internal static class XYFocusHelpers
     {
         return keyDeviceType switch
         {
-            null => true, // programmatic input, allow any subtree.
+            null => !modes.Equals(XYFocusNavigationModes.Disabled), // programmatic input, allow any subtree except Disabled.
             KeyDeviceType.Keyboard => modes.HasFlag(XYFocusNavigationModes.Keyboard),
             KeyDeviceType.Gamepad => modes.HasFlag(XYFocusNavigationModes.Gamepad),
             KeyDeviceType.Remote => modes.HasFlag(XYFocusNavigationModes.Remote),
             _ => throw new ArgumentOutOfRangeException(nameof(keyDeviceType), keyDeviceType, null)
         };
     }
+
+    internal static InputElement? FindXYSearchRoot(this InputElement visual, KeyDeviceType? keyDeviceType)
+    {
+        InputElement candidate = visual;
+        InputElement? candidateParent = visual.FindAncestorOfType<InputElement>();
+
+        while (candidateParent is not null && candidateParent.IsAllowedXYNavigationMode(keyDeviceType))
+        {
+            candidate = candidateParent;
+            candidateParent = candidate.FindAncestorOfType<InputElement>();
+        }
+
+        return candidate;
+    }
 }

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

@@ -418,4 +418,45 @@ public class KeyboardNavigationTests_XY : ScopedTestBase
 
         Assert.Equal(candidate, FocusManager.GetFocusManager(window)!.GetFocusedElement());
     }
+
+    [Fact]
+    public void Cannot_Focus_Across_XYFocus_Boundaries()
+    {
+        using var _ = UnitTestApplication.Start(TestServices.FocusableWindow);
+
+        var current = new Button() { Height = 20 };
+        var candidate = new Button() { Height = 20 };
+        var currentParent = new StackPanel
+        {
+            [XYFocus.NavigationModesProperty] = XYFocusNavigationModes.Enabled,
+            Orientation = Orientation.Vertical,
+            Spacing = 20,
+            Children = { current }
+        };
+        var candidateParent = new StackPanel
+        {
+            [XYFocus.NavigationModesProperty] = XYFocusNavigationModes.Enabled,
+            Orientation = Orientation.Vertical,
+            Spacing = 20,
+            Children = { candidate }
+        };
+
+        var grandparent = new StackPanel
+        {
+            [XYFocus.NavigationModesProperty] = XYFocusNavigationModes.Disabled,
+            Orientation = Orientation.Vertical,
+            Spacing = 20,
+            Children = { currentParent, candidateParent }
+        };
+
+        var window = new Window
+        {
+            [XYFocus.NavigationModesProperty] = XYFocusNavigationModes.Enabled,
+            Content = grandparent,
+            Height = 300
+        };
+        window.Show();
+
+        Assert.Null(KeyboardNavigationHandler.GetNext(current, NavigationDirection.Down));
+    }
 }