WIP

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -1162,9 +1162,9 @@ namespace Avalonia.Controls.Primitives
 
         private void InitializeSelectionModel(ISelectionModel model)
         {
-            if (_updateState is null && model.Source is null)
+            if (_updateState is null)
             {
-                model.Source = ItemsView;
+                model.Source = IsSet(ItemsSourceProperty) ? ItemsSource : Items;
             }
 
             model.PropertyChanged += OnSelectionModelPropertyChanged;

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

@@ -1441,7 +1441,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             var selection = new SelectionModel<string>();
             target.Selection = selection;
 
-            Assert.Same(target.ItemsView, selection.Source);
+            Assert.Same(target.ItemsSource, selection.Source);
         }
 
         [Fact]