Check ISelectable.IsSelected when items added.

Fixes #3181.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -383,12 +383,40 @@ protected override void ItemsChanged(AvaloniaPropertyChangedEventArgs e)
             }
 
             base.ItemsChanged(e);
+
+            var newItems = (IEnumerable)e.NewValue;
+
+            if (Items != null)
+            {
+                var index = 0;
+
+                foreach (var i in newItems)
+                {
+                    if (i is ISelectable s && s.IsSelected)
+                    {
+                        Selection.Select(index);
+                    }
+
+                    ++index;
+                }
+            }
         }
 
         /// <inheritdoc/>
         protected override void ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
         {
             base.ItemsCollectionChanged(sender, e);
+
+            if (e.NewItems != null)
+            {
+                for (var i = 0; i < e.NewItems.Count; ++i)
+                {
+                    if (e.NewItems[i] is ISelectable s && s.IsSelected)
+                    {
+                        Selection.Select(e.NewStartingIndex + i);
+                    }
+                }
+            }
         }
 
         /// <inheritdoc/>

tests/Avalonia.Controls.UnitTests/ComboBoxTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System.Collections.ObjectModel;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Shapes;
@@ -78,6 +79,33 @@ public void SelectionBoxItem_Rectangle_Is_Removed_From_Logical_Tree()
             Assert.False(((ILogical)rectangle).IsAttachedToLogicalTree);
         }
 
+        [Fact]
+        public void Selected_Item_Should_Be_Loaded_From_Inline_Items_IsSelected_Property()
+        {
+            var target = new ComboBox
+            {
+                Items = new[]
+                {
+                    new ComboBoxItem(),
+                    new ComboBoxItem { IsSelected = true },
+                }
+            };
+
+            Assert.Equal(1, target.SelectedIndex);
+        }
+
+        [Fact]
+        public void Selected_Item_Should_Be_Loaded_When_IsSelected_Inline_Item_Added()
+        {
+            var items = new ObservableCollection<ComboBoxItem>();
+            var target = new ComboBox { Items = items, };
+
+            items.Add(new ComboBoxItem());
+            items.Add(new ComboBoxItem { IsSelected = true });
+
+            Assert.Equal(1, target.SelectedIndex);
+        }
+
         private FuncControlTemplate GetTemplate()
         {
             return new FuncControlTemplate<ComboBox>((parent, scope) =>