Hackfix for calling SelectAll with SelectedItem binding (#13868)

* Added failing tests for #13676.

* Add hackfix for #13676.

---------

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

src/Avalonia.Controls/Selection/SelectionModel.cs

@@ -69,6 +69,18 @@ namespace Avalonia.Controls.Selection
             get => _selectedIndex;
             set
             {
+                if (_operation is not null && _operation.UpdateCount == 0)
+                {
+                    // An operation is in the process of being committed. In this case, if the new
+                    // value for SelectedIndex is unchanged then we need to ignore it. It could be
+                    // the result of a two-way binding to SelectedIndex writing back to the
+                    // property. The binding system should really be fixed to ensure that it's not
+                    // writing back the same value, but this is a workaround until the binding
+                    // refactor is complete. See #13676.
+                    if (value == _selectedIndex)
+                        return;
+                }
+
                 using var update = BatchUpdate();
                 Clear();
                 Select(value);
@@ -675,8 +687,6 @@ namespace Avalonia.Controls.Selection
                     }
                 }
                 
-                
-
                 if (raisePropertyChanged)
                 {
                     if (oldSelectedIndex != _selectedIndex)

tests/Avalonia.Controls.UnitTests/ListBoxTests_Multiple.cs

@@ -3,6 +3,7 @@ using System.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
+using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Styling;
@@ -577,6 +578,64 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(target.ContainerFromIndex(3).IsFocused);
         }
 
+        [Fact]
+        public void SelectAll_Works_From_No_Selection_When_SelectedItem_Is_Bound_TwoWay()
+        {
+            // Issue #13676
+            using var app = UnitTestApplication.Start(TestServices.RealFocus);
+            var target = new ListBox
+            {
+                Template = new FuncControlTemplate(CreateListBoxTemplate),
+                ItemsSource = new[] { "Foo", "Bar", "Baz", "Qux" },
+                SelectionMode = SelectionMode.Multiple,
+                Width = 100,
+                Height = 100,
+            };
+
+            var root = new TestRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            target.Bind(ListBox.SelectedItemProperty, new Binding("Tag") 
+            { 
+                Mode = BindingMode.TwoWay,
+                RelativeSource = new RelativeSource(RelativeSourceMode.Self),
+            });
+
+            target.SelectAll();
+
+            Assert.Equal(new[] { 0, 1, 2, 3 }, target.Selection.SelectedIndexes);
+            Assert.Equal(new[] { "Foo", "Bar", "Baz", "Qux" }, target.SelectedItems);
+        }
+
+        [Fact]
+        public void SelectAll_Works_From_No_Selection_When_SelectedIndex_Is_Bound_TwoWay()
+        {
+            // Issue #13676
+            using var app = UnitTestApplication.Start(TestServices.RealFocus);
+            var target = new ListBox
+            {
+                Template = new FuncControlTemplate(CreateListBoxTemplate),
+                ItemsSource = new[] { "Foo", "Bar", "Baz", "Qux" },
+                SelectionMode = SelectionMode.Multiple,
+                Width = 100,
+                Height = 100,
+            };
+
+            var root = new TestRoot(target);
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            target.Bind(ListBox.SelectedIndexProperty, new Binding("Tag")
+            {
+                Mode = BindingMode.TwoWay,
+                RelativeSource = new RelativeSource(RelativeSourceMode.Self),
+            });
+
+            target.SelectAll();
+
+            Assert.Equal(new[] { 0, 1, 2, 3 }, target.Selection.SelectedIndexes);
+            Assert.Equal(new[] { "Foo", "Bar", "Baz", "Qux" }, target.SelectedItems);
+        }
+
         private Control CreateListBoxTemplate(TemplatedControl parent, INameScope scope)
         {
             return new ScrollViewer