Fix TabItem's Content inheriting TabControl's DataContext instead of TabItem's (#20541)

* Child inherit DataContext from TabItem

* Remove whitespace changes

* Fix nullability error

* Fix using Julien's solution

src/Avalonia.Controls/TabControl.cs

@@ -1,4 +1,4 @@
-using System.Linq;
+using System.Diagnostics;
 using Avalonia.Collections;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Presenters;
@@ -7,7 +7,6 @@ using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
-using Avalonia.VisualTree;
 using Avalonia.Automation;
 using Avalonia.Controls.Metadata;
 using Avalonia.Reactive;
@@ -76,7 +75,7 @@ namespace Avalonia.Controls
             SelectionModeProperty.OverrideDefaultValue<TabControl>(SelectionMode.AlwaysSelected);
             ItemsPanelProperty.OverrideDefaultValue<TabControl>(DefaultPanel);
             AffectsMeasure<TabControl>(TabStripPlacementProperty);
-            SelectedItemProperty.Changed.AddClassHandler<TabControl>((x, e) => x.UpdateSelectedContent());
+            SelectedItemProperty.Changed.AddClassHandler<TabControl>((x, _) => x.UpdateSelectedContent());
             AutomationProperties.ControlTypeOverrideProperty.OverrideDefaultValue<TabControl>(AutomationControlType.Tab);
         }
 
@@ -231,7 +230,25 @@ namespace Avalonia.Controls
                     }
 
                     _selectedItemSubscriptions = new CompositeDisposable(
-                        container.GetObservable(ContentControl.ContentProperty).Subscribe(v => SelectedContent = v),
+                        container.GetObservable(ContentControl.ContentProperty).Subscribe(content =>
+                        {
+                            var contentElement = content as StyledElement;
+                            var contentDataContext = contentElement?.DataContext;
+                            SelectedContent = content;
+
+                            // When the ContentPresenter (ContentPart) displays content that is a Control, it doesn't
+                            // set its DataContext to that of the Control's. If the content doesn't set a DataContext,
+                            // then it gets inherited from the TabControl. Work around this issue by setting the
+                            // DataContext of the ContentPart to the content's original DataContext (inherited from
+                            // container).
+                            if (contentElement is not null &&
+                                contentElement.DataContext != contentDataContext &&
+                                ContentPart is not null)
+                            {
+                                Debug.Assert(!contentElement.IsSet(DataContextProperty));
+                                ContentPart.DataContext = contentDataContext;
+                            }
+                        }),
                         container.GetObservable(ContentControl.ContentTemplateProperty).Subscribe(v => SelectedContentTemplate = SelectContentTemplate(v)));
 
                     // Note how we fall back to our own ContentTemplate if the container doesn't specify one

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -9,6 +9,8 @@ using Avalonia.Controls.Selection;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
 using Avalonia.Data;
+using Avalonia.Harfbuzz;
+using Avalonia.Headless;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
@@ -259,51 +261,48 @@ namespace Avalonia.Controls.UnitTests
         [Fact]
         public void DataContexts_Should_Be_Correctly_Set()
         {
+            using var app = Start();
             var items = new object[]
             {
                 "Foo",
                 new Item("Bar"),
                 new TextBlock { Text = "Baz" },
                 new TabItem { Content = "Qux" },
-                new TabItem { Content = new TextBlock { Text = "Bob" } }
+                new TabItem { Content = new TextBlock { Text = "Bob" } },
+                new TabItem { DataContext = "Rob", Content = new TextBlock { Text = "Bob" } },
             };
 
             var target = new TabControl
             {
-                Template = TabControlTemplate(),
                 DataContext = "Base",
-                DataTemplates =
-                {
-                    new FuncDataTemplate<Item>((x, __) => new Button { Content = x })
-                },
                 ItemsSource = items,
             };
 
-            ApplyTemplate(target);
+            var root = CreateRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
 
-            target.ContentPart!.UpdateChild();
+            var dataContext = ((TextBlock)target.ContentPart!.Child!).DataContext;
-            var dataContext = ((TextBlock)target.ContentPart.Child!).DataContext;
             Assert.Equal(items[0], dataContext);
 
             target.SelectedIndex = 1;
-            target.ContentPart.UpdateChild();
             dataContext = ((Button)target.ContentPart.Child).DataContext;
             Assert.Equal(items[1], dataContext);
 
             target.SelectedIndex = 2;
-            target.ContentPart.UpdateChild();
             dataContext = ((TextBlock)target.ContentPart.Child).DataContext;
             Assert.Equal("Base", dataContext);
 
             target.SelectedIndex = 3;
-            target.ContentPart.UpdateChild();
             dataContext = ((TextBlock)target.ContentPart.Child).DataContext;
             Assert.Equal("Qux", dataContext);
 
             target.SelectedIndex = 4;
-            target.ContentPart.UpdateChild();
             dataContext = target.ContentPart.DataContext;
             Assert.Equal("Base", dataContext);
+
+            target.SelectedIndex = 5;
+            dataContext = target.ContentPart.Child.DataContext;
+            Assert.Equal("Rob", dataContext);
         }
 
         /// <summary>
@@ -843,6 +842,45 @@ namespace Avalonia.Controls.UnitTests
                 }.RegisterInNameScope(scope));
         }
 
+        private static ControlTheme CreateTabControlControlTheme()
+        {
+            return new ControlTheme(typeof(TabControl))
+            {
+                Setters =
+                {
+                    new Setter(TabControl.TemplateProperty, TabControlTemplate()),
+                },
+            };
+        }
+
+        private static ControlTheme CreateTabItemControlTheme()
+        {
+            return new ControlTheme(typeof(TabItem))
+            {
+                Setters =
+                {
+                    new Setter(TabItem.TemplateProperty, TabItemTemplate()),
+                },
+            };
+        }
+        
+        private static TestRoot CreateRoot(Control child)
+        {
+            return new TestRoot
+            {
+                Resources =
+                {
+                    { typeof(TabControl), CreateTabControlControlTheme() },
+                    { typeof(TabItem), CreateTabItemControlTheme() },
+                },
+                DataTemplates =
+                {
+                    new FuncDataTemplate<Item>((x, _) => new Button { Content = x.Value })
+                },
+                Child = child,
+            };
+        }
+
         private class TestTopLevel : TopLevel
         {
             private readonly ILayoutManager _layoutManager;
@@ -892,6 +930,19 @@ namespace Avalonia.Controls.UnitTests
             target.ContentPart!.ApplyTemplate();
         }
 
+        private IDisposable Start()
+        {
+            return UnitTestApplication.Start(
+                TestServices.MockThreadingInterface.With(
+                    fontManagerImpl: new HeadlessFontManagerStub(),
+                    keyboardDevice: () => new KeyboardDevice(),
+                    keyboardNavigation: () => new KeyboardNavigationHandler(),
+                    inputManager: new InputManager(),
+                    renderInterface: new HeadlessPlatformRenderInterface(),
+                    textShaperImpl: new HarfBuzzTextShaper(),
+                    assetLoader: new StandardAssetLoader()));
+        }
+
         private class Item
         {
             public Item(string value)