Make ExpandSubTree work again.

src/Avalonia.Controls/TreeView.cs

@@ -10,6 +10,7 @@ using Avalonia.Controls.Generators;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
+using Avalonia.Layout;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 
@@ -163,6 +164,9 @@ namespace Avalonia.Controls
         {
             item.IsExpanded = true;
 
+            if (item.Presenter?.Panel is null)
+                (this.GetVisualRoot() as ILayoutRoot)?.LayoutManager.ExecuteLayoutPass();
+
             if (item.Presenter?.Panel is { } panel)
             {
                 foreach (var child in panel.Children)