Update SelectedIndex when items removed before selection.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -316,26 +316,22 @@ namespace Avalonia.Controls.Primitives
 
                 case NotifyCollectionChangedAction.Remove:
                 case NotifyCollectionChangedAction.Replace:
-                    var selectedIndex = SelectedIndex;
-
-                    if (selectedIndex >= e.OldStartingIndex &&
-                        selectedIndex < e.OldStartingIndex + e.OldItems.Count)
+                    if (SelectedIndex >= e.OldStartingIndex && SelectedIndex < e.OldStartingIndex + e.OldItems.Count)
                     {
                         if (!AlwaysSelected)
                         {
-                            selectedIndex = SelectedIndex = -1;
+                            SelectedIndex = -1;
                         }
                         else
                         {
                             LostSelection();
                         }
                     }
-
-                    var items = Items?.Cast<object>();
-                    if (selectedIndex >= items.Count())
+                    else if (e.OldStartingIndex <= SelectedIndex)
                     {
-                        selectedIndex = SelectedIndex = items.Count() - 1;
+                        UpdateSelectedItem(SelectedIndex - e.OldItems.Count, false);
                     }
+
                     break;
 
                 case NotifyCollectionChangedAction.Move:

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

@@ -842,6 +842,32 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal("Bar", target.SelectedItem);
         }
 
+        [Fact]
+        public void Removing_Item_Before_SelectedItem_Should_Update_SelectedIndex()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "Baz"
+            };
+
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = items,
+                SelectedIndex = 1,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            items.RemoveAt(0);
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal("Bar", target.SelectedItem);
+        }
+
         private FuncControlTemplate Template()
         {
             return new FuncControlTemplate<SelectingItemsControl>(control =>

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

@@ -840,6 +840,34 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(new[] { "Foo", "Bar", "Baz", "Foo", "Bar", "Baz" }, target.SelectedItems);
         }
 
+        [Fact]
+        public void Adding_Item_Before_SelectedItems_Should_Update_Indexes()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "Baz"
+            };
+
+            var target = new ListBox
+            {
+                Template = Template(),
+                Items = items,
+                SelectionMode = SelectionMode.Multiple,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            target.SelectAll();
+            items.Insert(0, "Qux");
+
+            Assert.Equal(1, target.SelectedIndex);
+            Assert.Equal("Foo", target.SelectedItem);
+        }
+
+
         private IEnumerable<int> SelectedContainers(SelectingItemsControl target)
         {
             return target.Presenter.Panel.Children