Auto-select first item on `Reset`.

When a reset is received by `SelectingItemsControl` which has `AlwaysSelect == true`, select the first item if present.

Fixes #2754

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -333,6 +333,11 @@ namespace Avalonia.Controls.Primitives
                 case NotifyCollectionChangedAction.Move:
                 case NotifyCollectionChangedAction.Reset:
                     SelectedIndex = IndexOf(Items, SelectedItem);
+
+                    if (AlwaysSelected && SelectedIndex == -1 && ItemCount > 0)
+                    {
+                        SelectedIndex = 0;
+                    }
                     break;
             }
         }