Added failing test for #2754.

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

@@ -1,6 +1,8 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System.Collections.Generic;
+using System.Collections.Specialized;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
@@ -43,6 +45,24 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal("foo", target.SelectedItem);
         }
 
+
+        [Fact]
+        public void First_Item_Should_Be_Selected_When_Reset()
+        {
+            var items = new ResetOnAdd();
+            var target = new TestSelector
+            {
+                Items = items,
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            items.Add("foo");
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal("foo", target.SelectedItem);
+        }
+
         [Fact]
         public void Item_Should_Be_Selected_When_Selection_Removed()
         {
@@ -100,5 +120,18 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 SelectionModeProperty.OverrideDefaultValue<TestSelector>(SelectionMode.AlwaysSelected);
             }
         }
+
+        private class ResetOnAdd : List<string>, INotifyCollectionChanged
+        {
+            public event NotifyCollectionChangedEventHandler CollectionChanged;
+
+            public new void Add(string item)
+            {
+                base.Add(item);
+                CollectionChanged?.Invoke(
+                    this,
+                    new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
+            }
+        }
     }
 }