Fix clearing selection when clicking on selected item.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -999,26 +999,6 @@ namespace Avalonia.Controls.Primitives
                     SelectedItemProperty,
                     oldItem,
                     item);
-
-                UpdateSelectedItems(() =>
-                {
-                    if (clear)
-                    {
-                        SelectedItems.Clear();
-
-                        if (index != -1)
-                        {
-                            SelectedItems.Add(item);
-                        }
-                    }
-                    else
-                    {
-                        if (added != -1)
-                        {
-                            SelectedItems.Add(added);
-                        }
-                    }
-                });
             }
 
             if (removed != null && index != -1)
@@ -1028,6 +1008,8 @@ namespace Avalonia.Controls.Primitives
 
             if (added != -1 || removed?.Count > 0)
             {
+                ResetSelectedItems();
+
                 var e = new SelectionChangedEventArgs(
                     SelectionChangedEvent,
                     added != -1 ? new[] { ElementAt(Items, added) } : Array.Empty<object>(),

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

@@ -957,6 +957,30 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(new[] { "Foo", "Qux", "Baz" }, target.SelectedItems);
         }
 
+        [Fact]
+        public void Left_Click_On_SelectedItem_Should_Clear_Existing_Selection()
+        {
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = new[] { "Foo", "Bar", "Baz" },
+                ItemTemplate = new FuncDataTemplate<string>(x => new TextBlock { Width = 20, Height = 10 }),
+                SelectionMode = SelectionMode.Multiple,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+            target.SelectAll();
+
+            Assert.Equal(3, target.SelectedItems.Count);
+
+            _helper.Down((Interactive)target.Presenter.Panel.Children[0]);
+
+            Assert.Equal(1, target.SelectedItems.Count);
+            Assert.Equal(new[] { "Foo", }, target.SelectedItems);
+            Assert.Equal(new[] { 0 }, SelectedContainers(target));
+        }
+
         private IEnumerable<int> SelectedContainers(SelectingItemsControl target)
         {
             return target.Presenter.Panel.Children