Fix TabControl crash when rapidly switching tabs during transitions (#21227)

* Fix TabControl crash when switching back during tab transition

* More changes

src/Avalonia.Controls/TabControl.cs

@@ -254,13 +254,7 @@ namespace Avalonia.Controls
             _currentTransition = null;
             _shouldAnimate = false;
 
-            if (_contentPresenter2 is { IsVisible: true })
-            {
-                _contentPresenter2.IsVisible = false;
-                _contentPresenter2.Content = null;
-                _contentPresenter2.ContentTemplate = null;
-                _contentPresenter2.DataContext = null;
-            }
+            ResetTransitionPresenter(_contentPresenter2);
 
             int oldIndex = _previousSelectedIndex;
             _previousSelectedIndex = SelectedIndex;
@@ -306,8 +300,8 @@ namespace Avalonia.Controls
                             _contentPresenter2!.ContentTemplate = template;
                             _contentPresenter2!.IsVisible = true;
 
-                            if (content is Control && template is null)
-                                _contentPresenter2.SetContentWithDataContext(content, container.DataContext);
+                            if (content is Control control && template is null)
+                                SetControlContent(_contentPresenter2, control, container.DataContext);
                             else
                                 _contentPresenter2.Content = content;
 
@@ -320,8 +314,8 @@ namespace Avalonia.Controls
                             if (ContentPart != null)
                             {
                                 var template = SelectContentTemplate(container.GetValue(ContentControl.ContentTemplateProperty));
-                                if (content is Control && template is null)
-                                    ContentPart.SetContentWithDataContext(content, container.DataContext);
+                                if (content is Control control && template is null)
+                                    SetControlContent(ContentPart, control, container.DataContext);
                                 else
                                     ContentPart.Content = content;
                             }
@@ -408,6 +402,7 @@ namespace Avalonia.Controls
             _currentTransition?.Dispose();
             _currentTransition = null;
             _shouldAnimate = false;
+            ResetTransitionPresenter(_contentPresenter2);
         }
 
         protected override Size ArrangeOverride(Size finalSize)
@@ -496,6 +491,56 @@ namespace Avalonia.Controls
             (_contentPart, _contentPresenter2) = (_contentPresenter2, _contentPart);
         }
 
+        private void SetControlContent(ContentPresenter presenter, Control content, object? dataContext)
+        {
+            ClearOwningContentPresenter(presenter, content);
+            ClearPresenterContent(ContentPart, presenter, content);
+            ClearPresenterContent(_contentPresenter2, presenter, content);
+            presenter.SetContentWithDataContext(content, dataContext);
+        }
+
+        private void ClearOwningContentPresenter(ContentPresenter targetPresenter, Control content)
+        {
+            if (content.VisualParent is ContentPresenter parent &&
+                !ReferenceEquals(parent, targetPresenter) &&
+                ReferenceEquals(parent.Host, this) &&
+                ReferenceEquals(parent.Content, content))
+            {
+                ClearPresenterContent(parent, content);
+            }
+        }
+
+        private static void ClearPresenterContent(ContentPresenter? presenter, ContentPresenter targetPresenter, Control content)
+        {
+            if (!ReferenceEquals(presenter, targetPresenter))
+                ClearPresenterContent(presenter, content);
+        }
+
+        private static void ClearPresenterContent(ContentPresenter? presenter, Control content)
+        {
+            if (presenter != null && ReferenceEquals(presenter.Content, content))
+            {
+                presenter.Content = null;
+                presenter.ContentTemplate = null;
+                presenter.DataContext = null;
+                presenter.RenderTransform = null;
+                presenter.Opacity = 1;
+            }
+        }
+
+        private static void ResetTransitionPresenter(ContentPresenter? presenter)
+        {
+            if (presenter == null)
+                return;
+
+            presenter.IsVisible = false;
+            presenter.Content = null;
+            presenter.ContentTemplate = null;
+            presenter.DataContext = null;
+            presenter.RenderTransform = null;
+            presenter.Opacity = 1;
+        }
+
         private void UpdateTabStripPlacement()
         {
             var controls = ItemsPresenterPart?.Panel?.Children;

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
@@ -21,6 +22,7 @@ using Avalonia.Markup.Xaml;
 using Avalonia.Platform;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
+using Avalonia.VisualTree;
 using Moq;
 using Xunit;
 
@@ -913,6 +915,165 @@ namespace Avalonia.Controls.UnitTests
                 Times.Once);
         }
 
+        [Fact]
+        public void Interrupted_PageTransition_Can_Select_Original_Control_Before_Previous_Transition_Completes()
+        {
+            using var app = Start();
+
+            var firstPage = new ContentPage { Content = "Alpha" };
+            var secondPage = new ContentPage { Content = "Beta" };
+            var starts = new List<(object? FromContent, object? ToContent, bool Forward)>();
+            var transitionGate = new TaskCompletionSource();
+            var transition = new Mock<IPageTransition>();
+            transition
+                .Setup(t => t.Start(
+                    It.IsAny<Visual?>(), It.IsAny<Visual?>(),
+                    It.IsAny<bool>(), It.IsAny<CancellationToken>()))
+                .Callback<Visual?, Visual?, bool, CancellationToken>((from, to, forward, _) =>
+                {
+                    starts.Add((
+                        (from as ContentPresenter)?.Content,
+                        (to as ContentPresenter)?.Content,
+                        forward));
+                })
+                .Returns(transitionGate.Task);
+
+            var target = new TabControl
+            {
+                PageTransition = transition.Object,
+                Items =
+                {
+                    new TabItem { Name = "first", Content = firstPage },
+                    new TabItem { Name = "second", Content = secondPage },
+                },
+            };
+
+            var root = CreateRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            target.SelectedIndex = 1;
+            root.LayoutManager.ExecuteLayoutPass();
+
+            Assert.Single(starts);
+            Assert.Same(firstPage, starts[0].FromContent);
+            Assert.Same(secondPage, starts[0].ToContent);
+            Assert.True(starts[0].Forward);
+
+            var exception = Record.Exception(() => target.SelectedIndex = 0);
+
+            Assert.Null(exception);
+
+            root.LayoutManager.ExecuteLayoutPass();
+
+            Assert.Equal(2, starts.Count);
+            Assert.Null(starts[1].FromContent);
+            Assert.Same(firstPage, starts[1].ToContent);
+            Assert.False(starts[1].Forward);
+            Assert.Same(firstPage, target.SelectedContent);
+        }
+
+        [Fact]
+        public void Pending_PageTransition_Can_Select_Original_Control_Before_Transition_Starts()
+        {
+            using var app = Start();
+
+            var firstPage = new ContentPage { Content = "Alpha" };
+            var secondPage = new ContentPage { Content = "Beta" };
+            var starts = new List<(object? FromContent, object? ToContent, bool Forward)>();
+            var transition = new Mock<IPageTransition>();
+            transition
+                .Setup(t => t.Start(
+                    It.IsAny<Visual?>(), It.IsAny<Visual?>(),
+                    It.IsAny<bool>(), It.IsAny<CancellationToken>()))
+                .Callback<Visual?, Visual?, bool, CancellationToken>((from, to, forward, _) =>
+                {
+                    starts.Add((
+                        (from as ContentPresenter)?.Content,
+                        (to as ContentPresenter)?.Content,
+                        forward));
+                })
+                .Returns(Task.CompletedTask);
+
+            var target = new TabControl
+            {
+                PageTransition = transition.Object,
+                Items =
+                {
+                    new TabItem { Name = "first", Content = firstPage },
+                    new TabItem { Name = "second", Content = secondPage },
+                },
+            };
+
+            var root = CreateRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            target.SelectedIndex = 1;
+            var exception = Record.Exception(() => target.SelectedIndex = 0);
+
+            Assert.Null(exception);
+
+            root.LayoutManager.ExecuteLayoutPass();
+
+            Assert.Single(starts);
+            Assert.Null(starts[0].FromContent);
+            Assert.Same(firstPage, starts[0].ToContent);
+            Assert.False(starts[0].Forward);
+            Assert.Same(firstPage, target.SelectedContent);
+        }
+
+        [Fact]
+        public void Interrupted_PageTransition_Clears_Reused_Control_From_Owning_SelectedContentHost()
+        {
+            using var app = Start();
+
+            var firstPage = new ContentPage { Content = "Alpha" };
+            var secondPage = new ContentPage { Content = "Beta" };
+            var transition = new Mock<IPageTransition>();
+            transition
+                .Setup(t => t.Start(
+                    It.IsAny<Visual?>(), It.IsAny<Visual?>(),
+                    It.IsAny<bool>(), It.IsAny<CancellationToken>()))
+                .Returns(Task.CompletedTask);
+
+            var target = new TabControl
+            {
+                Items =
+                {
+                    new TabItem { Name = "first", Content = firstPage },
+                    new TabItem { Name = "second", Content = secondPage },
+                },
+            };
+
+            var root = CreateRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            target.SelectedIndex = 1;
+            root.LayoutManager.ExecuteLayoutPass();
+
+            var primary = target.GetVisualDescendants()
+                .OfType<ContentPresenter>()
+                .Single(x => x.Name == "PART_SelectedContentHost");
+            var secondary = target.GetVisualDescendants()
+                .OfType<ContentPresenter>()
+                .Single(x => x.Name == "PART_SelectedContentHost2");
+
+            // Simulate the stale presenter ownership that can happen when tab changes
+            // interrupt a transition: the page is still parented by the named content
+            // host, but the active field no longer points at that host.
+            primary.SetContentWithDataContext(firstPage, null);
+            secondary.IsVisible = false;
+            SetPrivateField(target, "_contentPart", secondary);
+            SetPrivateField(target, "_contentPresenter2", secondary);
+
+            target.PageTransition = transition.Object;
+            var exception = Record.Exception(() => target.SelectedIndex = 0);
+
+            Assert.Null(exception);
+            Assert.Same(firstPage, target.SelectedContent);
+            Assert.Null(primary.Content);
+            Assert.Same(firstPage, secondary.Content);
+        }
+
         private static IControlTemplate TabControlTemplate()
         {
             return new FuncControlTemplate<TabControl>((parent, scope) =>
@@ -943,6 +1104,13 @@ namespace Avalonia.Controls.UnitTests
                 });
         }
 
+        private static void SetPrivateField<T>(TabControl target, string name, T value)
+        {
+            var field = typeof(TabControl).GetField(name, BindingFlags.Instance | BindingFlags.NonPublic);
+            Assert.NotNull(field);
+            field.SetValue(target, value);
+        }
+
         private static IControlTemplate TabItemTemplate()
         {
             return new FuncControlTemplate<TabItem>((parent, scope) =>