Fix scrolling to selection.

A few `AutoScrollToSelectedItem` improvements:

- Scroll to current selected item when it's set to true
- Scroll to current selected item when list first displayed
- Scroll to current selected item when attached to visual tree if the selection was changed while it wasn't attached

Fixes #4100

src/Avalonia.Controls/ListBox.cs

@@ -163,6 +163,7 @@ namespace Avalonia.Controls
 
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
+            base.OnApplyTemplate(e);
             Scroll = e.NameScope.Find<IScrollable>("PART_ScrollViewer");
         }
     }

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -116,6 +116,7 @@ namespace Avalonia.Controls.Primitives
         private IList? _oldSelectedItems;
         private bool _ignoreContainerSelectionChanged;
         private UpdateState? _updateState;
+        private bool _hasScrolledToSelectedItem;
 
         /// <summary>
         /// Initializes static members of the <see cref="SelectingItemsControl"/> class.
@@ -381,6 +382,28 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToVisualTree(e);
+            AutoScrollToSelectedItemIfNecessary();
+        }
+
+        protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+        {
+            base.OnApplyTemplate(e);
+
+            void ExecuteScrollWhenLayoutUpdated(object sender, EventArgs e)
+            {
+                LayoutUpdated -= ExecuteScrollWhenLayoutUpdated;
+                AutoScrollToSelectedItemIfNecessary();
+            }
+
+            if (AutoScrollToSelectedItem)
+            {
+                LayoutUpdated += ExecuteScrollWhenLayoutUpdated;
+            }
+        }
+
         /// <inheritdoc/>
         protected override void OnContainersMaterialized(ItemContainerEventArgs e)
         {
@@ -481,6 +504,10 @@ namespace Avalonia.Controls.Primitives
         {
             base.OnPropertyChanged(change);
 
+            if (change.Property == AutoScrollToSelectedItemProperty)
+            {
+                AutoScrollToSelectedItemIfNecessary();
+            }
             if (change.Property == ItemsProperty && _updateState is null && _selection is object)
             {
                 var newValue = change.NewValue.GetValueOrDefault<IEnumerable>();
@@ -669,12 +696,10 @@ namespace Avalonia.Controls.Primitives
         /// <param name="e">The event args.</param>
         private void OnSelectionModelPropertyChanged(object sender, PropertyChangedEventArgs e)
         {
-            if (e.PropertyName == nameof(ISelectionModel.AnchorIndex) && AutoScrollToSelectedItem)
+            if (e.PropertyName == nameof(ISelectionModel.AnchorIndex))
             {
-                if (Selection.AnchorIndex > 0)
-                {
-                    ScrollIntoView(Selection.AnchorIndex);
-                }
+                _hasScrolledToSelectedItem = false;
+                AutoScrollToSelectedItemIfNecessary();
             }
             else if (e.PropertyName == nameof(ISelectionModel.SelectedIndex) && _oldSelectedIndex != SelectedIndex)
             {
@@ -751,6 +776,19 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        private void AutoScrollToSelectedItemIfNecessary()
+        {
+            if (AutoScrollToSelectedItem &&
+                !_hasScrolledToSelectedItem &&
+                Presenter is object &&
+                Selection.AnchorIndex > 0 &&
+                ((IVisual)this).IsAttachedToVisualTree)
+            {
+                ScrollIntoView(Selection.AnchorIndex);
+                _hasScrolledToSelectedItem = true;
+            }
+        }
+
         /// <summary>
         /// Called when a container raises the <see cref="IsSelectedChangedEvent"/>.
         /// </summary>

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests.cs

@@ -1402,12 +1402,36 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 Items = items,
             };
 
+            var raised = false;
+
             Prepare(target);
+            target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
+            target.SelectedIndex = 2;
+
+            Assert.True(raised);
+        }
+
+        [Fact]
+        public void AutoScrollToSelectedItem_Causes_Scroll_To_Initial_SelectedItem()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "Baz"
+            };
+
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = items,
+            };
 
             var raised = false;
-            target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
 
+            target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
             target.SelectedIndex = 2;
+            Prepare(target);
 
             Assert.True(raised);
         }
@@ -1451,6 +1475,99 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }
         }
 
+        [Fact]
+        public void AutoScrollToSelectedItem_Scrolls_When_Reattached_To_Visual_Tree_If_Selection_Changed_While_Detached_From_Visual_Tree()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "Baz"
+            };
+
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = items,
+                SelectedIndex = 2,
+            };
+
+            var raised = false;
+
+            Prepare(target);
+
+            var root = (TestRoot)target.Parent;
+
+            target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
+
+            root.Child = null;
+            target.SelectedIndex = 1;
+            root.Child = target;
+
+            Assert.True(raised);
+        }
+
+        [Fact]
+        public void AutoScrollToSelectedItem_Doesnt_Scroll_If_Reattached_To_Visual_Tree_With_No_Selection_Change()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "Baz"
+            };
+
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = items,
+                SelectedIndex = 2,
+            };
+
+            var raised = false;
+
+            Prepare(target);
+
+            var root = (TestRoot)target.Parent;
+
+            target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
+
+            root.Child = null;
+            root.Child = target;
+
+            Assert.False(raised);
+        }
+
+        [Fact]
+        public void AutoScrollToSelectedItem_Causes_Scroll_When_Turned_On()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "Baz"
+            };
+
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = items,
+                AutoScrollToSelectedItem = false,
+            };
+
+            Prepare(target);
+
+            var raised = false;
+            target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
+            target.SelectedIndex = 2;
+
+            Assert.False(raised);
+
+            target.AutoScrollToSelectedItem = true;
+
+            Assert.True(raised);
+        }
+
         [Fact]
         public void Can_Set_Both_SelectedItem_And_SelectedItems_During_Initialization()
         {