Added failing tests for TabControl tab navigation.

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -5,8 +5,10 @@ using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Selection;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
+using Avalonia.Input;
 using Avalonia.LogicalTree;
 using Avalonia.Markup.Xaml;
 using Avalonia.Styling;
@@ -413,6 +415,117 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Tab_Navigation_Should_Move_To_First_TabItem_When_No_Anchor_Element_Selected()
+        {
+            var services = TestServices.StyledWindow.With(
+                focusManager: new FocusManager(),
+                keyboardDevice: () => new KeyboardDevice());
+            using var app = UnitTestApplication.Start(services);
+
+            var target = new TabControl
+            {
+                Template = TabControlTemplate(),
+                Items =
+                {
+                    new TabItem { Header = "foo" },
+                    new TabItem { Header = "bar" },
+                    new TabItem { Header = "baz" },
+                }
+            };
+
+            var button = new Button
+            {
+                Content = "Button",
+                [DockPanel.DockProperty] = Dock.Top,
+            };
+
+            var root = new TestRoot
+            {
+                Child = new DockPanel
+                {
+                    Children =
+                    {
+                        button,
+                        target,
+                    }
+                }
+            };
+
+            var navigation = new KeyboardNavigationHandler();
+            navigation.SetOwner(root);
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            button.Focus();
+            RaiseKeyEvent(button, Key.Tab);
+
+            var item = target.ContainerFromIndex(0);
+            Assert.Same(item, FocusManager.Instance.Current);
+        }
+
+        [Fact]
+        public void Tab_Navigation_Should_Move_To_Anchor_TabItem()
+        {
+            var services = TestServices.StyledWindow.With(
+                focusManager: new FocusManager(),
+                keyboardDevice: () => new KeyboardDevice());
+            using var app = UnitTestApplication.Start(services);
+
+            var target = new TestTabControl
+            {
+                Template = TabControlTemplate(),
+                Items =
+                {
+                    new TabItem { Header = "foo" },
+                    new TabItem { Header = "bar" },
+                    new TabItem { Header = "baz" },
+                }
+            };
+
+            var button = new Button
+            {
+                Content = "Button",
+                [DockPanel.DockProperty] = Dock.Top,
+            };
+
+            var root = new TestRoot
+            {
+                Width = 1000,
+                Height = 1000,
+                Child = new DockPanel
+                {
+                    Children =
+                    {
+                        button,
+                        target,
+                    }
+                }
+            };
+
+            var navigation = new KeyboardNavigationHandler();
+            navigation.SetOwner(root);
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            button.Focus();
+            target.Selection.AnchorIndex = 1;
+            RaiseKeyEvent(button, Key.Tab);
+
+            var item = target.ContainerFromIndex(1);
+            Assert.Same(item, FocusManager.Instance.Current);
+
+            RaiseKeyEvent(item, Key.Tab);
+
+            Assert.Same(button, FocusManager.Instance.Current);
+
+            target.Selection.AnchorIndex = 2;
+            RaiseKeyEvent(button, Key.Tab);
+
+            item = target.ContainerFromIndex(2);
+            Assert.Same(item, FocusManager.Instance.Current);
+        }
+
         private static IControlTemplate TabControlTemplate()
         {
             return new FuncControlTemplate<TabControl>((parent, scope) =>
@@ -452,6 +565,16 @@ namespace Avalonia.Controls.UnitTests
             target.Arrange(new Rect(target.DesiredSize));
         }
 
+        private static void RaiseKeyEvent(Control target, Key key, KeyModifiers inputModifiers = 0)
+        {
+            target.RaiseEvent(new KeyEventArgs
+            {
+                RoutedEvent = InputElement.KeyDownEvent,
+                KeyModifiers = inputModifiers,
+                Key = key
+            });
+        }
+
         private static void ApplyTemplate(TabControl target)
         {
             target.ApplyTemplate();
@@ -479,5 +602,11 @@ namespace Avalonia.Controls.UnitTests
 
             public string Value { get; }
         }
+
+        private class TestTabControl : TabControl, IStyleable
+        {
+            Type IStyleable.StyleKey => typeof(TabControl);
+            public new ISelectionModel Selection => base.Selection;
+        }
     }
 }