Fix AlwaysSelected in conjuction with SelectedItem/SelectedIndex binding (#14107)

* Added failing tests for #12733.

* Clear SkipLostSelection on batch update start.

If `Source` is changed during a collection update, then the `Clear()` operation will not be committed immediately due to `_isSourceCollectionChanging` being set. In this case, `update.Operation` will still have `SkipLostSelection == true`, meaning that `LostSelection` will not be raised, causing #12733. Clear the flag manually each time `BeginBatchUpdate` is called to avoid this.

Fixes #12733

---------

Co-authored-by: Max Katz <maxkatz6@outlook.com>

src/Avalonia.Controls/Selection/SelectionModel.cs

@@ -203,6 +203,7 @@ namespace Avalonia.Controls.Selection
         public void BeginBatchUpdate()
         {
             _operation ??= new Operation(this);
+            _operation.SkipLostSelection = false;
             ++_operation.UpdateCount;
         }
 

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

@@ -2292,6 +2292,25 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         }
 
+        [Fact]
+        public void Changing_DataContext_Respects_AlwaysSelected()
+        {
+            // Issue #12733
+            var target = new ListBox 
+            { 
+                DataContext = Enumerable.Range(0, 10).ToList(),
+                SelectionMode = SelectionMode.AlwaysSelected,
+                Template = Template(),
+                [!ListBox.ItemsSourceProperty] = new Binding(),
+            };
+
+            Assert.Equal(0, target.SelectedIndex);
+
+            target.DataContext = Enumerable.Range(10, 10).ToList();
+
+            Assert.Equal(0, target.SelectedIndex);
+        }
+
         [Theory]
         [MemberData(nameof(GetSelectionFieldPermutationParameters))]
         public void SelectedItem_And_Selection_Properties_Work_In_Any_Order_When_Initializing(SelectionField[] fields)

tests/Avalonia.Controls.UnitTests/Selection/SelectionModelTests_Single.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections;
 using System.Collections.Specialized;
 using System.Linq;
 using Avalonia.Collections;
@@ -1346,6 +1347,38 @@ namespace Avalonia.Controls.UnitTests.Selection
 
                 Assert.Equal(0, raised);
             }
+
+            [Fact]
+            public void LostSelection_Is_Called_When_Source_Changed_While_CollectionChange_In_Progress()
+            {
+                // Issue #12733.
+                var data1 = new AvaloniaList<string> { "foo1", "bar1", "baz1" };
+                var data2 = new AvaloniaList<string> { "foo1", "bar1", "baz1" };
+                var target = new DerivedSelectionModel { Source = data1 };
+                var raised = 0;
+
+                target.LostSelection += (s, e) =>
+                {
+                    if (target.Source == data2)
+                    {
+                        ++raised;
+                    }
+                };
+
+                target.UpdateSource(data2);
+
+                Assert.Equal(1, raised);
+            }
+
+            private class DerivedSelectionModel : SelectionModel<string?>
+            {
+                public void UpdateSource(IEnumerable? source)
+                {
+                    OnSourceCollectionChangeStarted();
+                    Source = source;
+                    OnSourceCollectionChangeFinished();
+                }
+            }
         }
 
         public class UntypedInterface