Merge pull request #3047 from Gillibald/fixes/2987failingTest

Properly handle setting SelectedIndex from XAML

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -304,6 +304,11 @@ namespace Avalonia.Controls.Primitives
         {
             base.ItemsCollectionChanged(sender, e);
 
+            if (_updateCount > 0)
+            {
+                return;
+            }
+
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
@@ -1071,13 +1076,20 @@ namespace Avalonia.Controls.Primitives
 
         private void UpdateFinished()
         {
-            if (_updateSelectedIndex != int.MinValue)
+            if (_updateSelectedItem != null)
             {
-                SelectedIndex = _updateSelectedIndex;
+                SelectedItem = _updateSelectedItem;
             }
-            else if (_updateSelectedItem != null)
+            else
             {
-                SelectedItem = _updateSelectedItem;
+                if (ItemCount == 0 && SelectedIndex != -1)
+                {
+                    SelectedIndex = -1;
+                }
+                else
+                {
+                    SelectedIndex = _updateSelectedIndex != int.MinValue ? _updateSelectedIndex : 0;
+                }
             }
         }
 

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

@@ -109,6 +109,28 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.True(items[1].IsSelected);
         }
 
+        [Fact]
+        public void Setting_SelectedIndex_During_Initialize_Should_Select_Item_When_AlwaysSelected_Is_Used()
+        {
+            var listBox = new ListBox
+            {
+                SelectionMode = SelectionMode.Single | SelectionMode.AlwaysSelected
+            };
+
+            listBox.BeginInit();
+
+            listBox.SelectedIndex = 1;
+            var items = new AvaloniaList<string>();
+            listBox.Items = items;
+            items.Add("A");
+            items.Add("B");
+            items.Add("C");
+
+            listBox.EndInit();
+
+            Assert.Equal("B", listBox.SelectedItem);
+        }
+
         [Fact]
         public void Setting_SelectedIndex_Before_ApplyTemplate_Should_Set_Item_IsSelected_True()
         {