Add focus-visible support with tests

src/Avalonia.Input/InputElement.cs

@@ -158,6 +158,7 @@ namespace Avalonia.Input
 
         private bool _isEffectivelyEnabled = true;
         private bool _isFocused;
+        private bool _isFocusVisible;
         private bool _isPointerOver;
         private GestureRecognizerCollection _gestureRecognizers;
 
@@ -427,7 +428,9 @@ namespace Avalonia.Input
         /// <param name="e">The event args.</param>
         protected virtual void OnGotFocus(GotFocusEventArgs e)
         {
-            IsFocused = e.Source == this;
+            var isFocused = e.Source == this;
+            _isFocusVisible = isFocused && (e.NavigationMethod == NavigationMethod.Directional || e.NavigationMethod == NavigationMethod.Tab);
+            IsFocused = isFocused;
         }
 
         /// <summary>
@@ -436,6 +439,7 @@ namespace Avalonia.Input
         /// <param name="e">The event args.</param>
         protected virtual void OnLostFocus(RoutedEventArgs e)
         {
+            _isFocusVisible = false;
             IsFocused = false;
         }
 
@@ -602,6 +606,7 @@ namespace Avalonia.Input
             if (isFocused.HasValue)
             {
                 PseudoClasses.Set(":focus", isFocused.Value);
+                PseudoClasses.Set(":focus-visible", _isFocusVisible);
             }
             
             if (isPointerOver.HasValue)

tests/Avalonia.Input.UnitTests/InputElement_Focus.cs

@@ -42,5 +42,84 @@ namespace Avalonia.Input.UnitTests
                 Assert.Null(FocusManager.Instance.Current);
             }
         }
+
+        [Fact]
+        public void Focus_Pseudoclass_Should_Be_Applied_On_Focus()
+        {
+            using (UnitTestApplication.Start(TestServices.RealFocus))
+            {
+                var target1 = new Decorator();
+                var target2 = new Decorator();
+                var root = new TestRoot
+                {
+                    Child = new StackPanel
+                    {
+                        Children =
+                        {
+                            target1,
+                            target2
+                        }
+                    }
+                };
+
+                target1.ApplyTemplate();
+                target2.ApplyTemplate();
+
+
+                FocusManager.Instance?.Focus(target1);
+                Assert.True(target1.IsFocused);
+                Assert.True(target1.Classes.Contains(":focus"));
+                Assert.False(target2.IsFocused);
+                Assert.False(target2.Classes.Contains(":focus"));
+
+                FocusManager.Instance?.Focus(target2, NavigationMethod.Tab);
+                Assert.False(target1.IsFocused);
+                Assert.False(target1.Classes.Contains(":focus"));
+                Assert.True(target2.IsFocused);
+                Assert.True(target2.Classes.Contains(":focus"));
+            }
+        }
+
+        [Fact]
+        public void Control_FocusVsisible_Pseudoclass_Should_Be_Applied_On_Tab_And_DirectionalFocus()
+        {
+            using (UnitTestApplication.Start(TestServices.RealFocus))
+            {
+                var target1 = new Decorator();
+                var target2 = new Decorator();
+                var root = new TestRoot
+                {
+                    Child = new StackPanel
+                    {
+                        Children =
+                        {
+                            target1,
+                            target2
+                        }
+                    }
+                };
+
+                target1.ApplyTemplate();
+                target2.ApplyTemplate();
+
+                FocusManager.Instance?.Focus(target1);
+                Assert.True(target1.IsFocused);
+                Assert.False(target1.Classes.Contains(":focus-visible"));
+                Assert.False(target2.IsFocused);
+                Assert.False(target2.Classes.Contains(":focus-visible"));
+
+                FocusManager.Instance?.Focus(target2, NavigationMethod.Tab);
+                Assert.False(target1.IsFocused);
+                Assert.False(target1.Classes.Contains(":focus-visible"));
+                Assert.True(target2.IsFocused);
+                Assert.True(target2.Classes.Contains(":focus-visible"));
+
+                FocusManager.Instance?.Focus(target1, NavigationMethod.Directional);
+                Assert.True(target1.IsFocused);
+                Assert.True(target1.Classes.Contains(":focus-visible"));
+                Assert.False(target2.IsFocused);
+                Assert.False(target2.Classes.Contains(":focus-visible"));
+            }
+        }
     }
 }