Fix scroll to selected item logic.

- Use `TreeContainerFromItem` when trying to find the container for the selected item, it can be anywhere in the tree
- Only scroll to `SelectedItem`, not another item in `SelectedItems`

src/Avalonia.Controls/TreeView.cs

@@ -358,9 +358,13 @@ namespace Avalonia.Controls
 
                     SelectedItemsAdded(e.NewItems!.Cast<object>().ToArray());
 
-                    if (AutoScrollToSelectedItem)
+                    var selectedItem = SelectedItem;
+
+                    if (AutoScrollToSelectedItem && 
+                        selectedItem is not null &&
+                        e.NewItems![0] == selectedItem)
                     {
-                        var container = ContainerFromItem(e.NewItems![0]!);
+                        var container = TreeContainerFromItem(selectedItem);
 
                         container?.BringIntoView();
                     }