mus: Do not notify clients when the roots are reordered.

components/mus/ws/window_tree_impl.cc

Index: components/mus/ws/window_tree_impl.cc
diff --git a/components/mus/ws/window_tree_impl.cc b/components/mus/ws/window_tree_impl.cc
index 80690ca809f36efd813414863b43e11d8419696e..71532ce300ceef9cdd029eda2890a519ea2d336a 100644
--- a/components/mus/ws/window_tree_impl.cc
+++ b/components/mus/ws/window_tree_impl.cc
@@ -409,11 +409,18 @@ void WindowTreeImpl::ProcessWindowReorder(const ServerWindow* window,
                                           const ServerWindow* relative_window,
                                           mojom::OrderDirection direction,
                                           bool originated_change) {
+  DCHECK_EQ(window->parent(), relative_window->parent());
   if (originated_change || !IsWindowKnown(window) ||
       !IsWindowKnown(relative_window) ||
       connection_manager_->DidConnectionMessageClient(id_))
     return;
 
+  // Do not notify ordering changes of the root windows, since the client
+  // doesn't know about the ancestors of the roots, and so can't do anything
+  // about this ordering change of the root.
+  if (HasRoot(window) || HasRoot(relative_window))
+    return;
+
   client_->OnWindowReordered(MapWindowIdToClient(window),
                              MapWindowIdToClient(relative_window), direction);
   connection_manager_->OnConnectionMessagedClient(id_);