Add some unit tests for FocusWithin

tests/Avalonia.Input.UnitTests/InputElement_Focus.cs

@@ -121,5 +121,93 @@ namespace Avalonia.Input.UnitTests
                 Assert.False(target2.Classes.Contains(":focus-visible"));
             }
         }
+        
+        [Fact]
+        public void Control_FocusWithin_PseudoClass_Should_Be_Applied()
+        {
+            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-within"));
+                Assert.True(target1.IsKeyboardFocusWithin);
+                Assert.True(root.Child.Classes.Contains(":focus-within"));
+                Assert.True(root.Child.IsKeyboardFocusWithin);
+                Assert.True(root.Classes.Contains(":focus-within"));
+                Assert.True(root.IsKeyboardFocusWithin);
+            }
+        }
+        
+        [Fact]
+        public void Control_FocusWithin_PseudoClass_Should_Be_Applied_and_Removed()
+        {
+            using (UnitTestApplication.Start(TestServices.RealFocus))
+            {
+                var target1 = new Decorator();
+                var target2 = new Decorator();
+                var panel1 = new Panel { Children = { target1 } };
+                var panel2 = new Panel { Children = { target2 } };
+                var root = new TestRoot
+                {
+                    Child = new StackPanel
+                    {
+                        Children =
+                        {
+                            panel1,
+                            panel2
+                        }
+                    }
+                };
+                
+                target1.ApplyTemplate();
+                target2.ApplyTemplate();
+
+                FocusManager.Instance?.Focus(target1);
+                Assert.True(target1.IsFocused);
+                Assert.True(target1.Classes.Contains(":focus-within"));
+                Assert.True(target1.IsKeyboardFocusWithin);
+                Assert.True(panel1.Classes.Contains(":focus-within"));
+                Assert.True(panel1.IsKeyboardFocusWithin);
+                Assert.True(root.Child.Classes.Contains(":focus-within"));
+                Assert.True(root.Child.IsKeyboardFocusWithin);
+                Assert.True(root.Classes.Contains(":focus-within"));
+                Assert.True(root.IsKeyboardFocusWithin);
+                
+                FocusManager.Instance?.Focus(target2);
+                
+                Assert.False(target1.IsFocused);
+                Assert.False(target1.Classes.Contains(":focus-within"));
+                Assert.False(target1.IsKeyboardFocusWithin);
+                Assert.False(panel1.Classes.Contains(":focus-within"));
+                Assert.False(panel1.IsKeyboardFocusWithin);
+                Assert.True(root.Child.Classes.Contains(":focus-within"));
+                Assert.True(root.Child.IsKeyboardFocusWithin);
+                Assert.True(root.Classes.Contains(":focus-within"));
+                Assert.True(root.IsKeyboardFocusWithin);
+                
+                Assert.True(target2.IsFocused);
+                Assert.True(target2.Classes.Contains(":focus-within"));
+                Assert.True(target2.IsKeyboardFocusWithin);
+                Assert.True(panel2.Classes.Contains(":focus-within"));
+                Assert.True(panel2.IsKeyboardFocusWithin);
+            }
+        }
     }
 }