mus: Fix ash::GetRootWindowController for child windows/widgets

ash/root_window_controller.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/root_window_controller.h"
 
 #include <queue>
 #include <vector>
 
 #include "ash/ash_constants.h"
@@ skipping 1038 matching lines @@
 
 void RootWindowController::OnTouchHudProjectionToggled(bool enabled) {
   if (enabled)
     EnableTouchHudProjection();
   else
     DisableTouchHudProjection();
 }
 
 RootWindowController* GetRootWindowController(
     const aura::Window* root_window) {
-  return root_window ? GetRootWindowSettings(root_window)->controller : NULL;
+  if (!root_window)
+    return nullptr;
+
+  if (Shell::GetInstance()->in_mus()) {
+    // On mus, like desktop aura, each top-level widget has its own root window,
+    // so |root_window| is not necessarily the display's root. For now, just
+    // the use the primary display root.
+    // TODO(jamescook): Multi-display support. This depends on how mus windows
+    // will be owned by displays.
+    aura::Window* primary_root_window = Shell::GetInstance()
+                                            ->window_tree_host_manager()
+                                            ->GetPrimaryRootWindow();
+    return GetRootWindowSettings(primary_root_window)->controller;
+  }
+
+  return GetRootWindowSettings(root_window)->controller;
 }
 
 }  // namespace ash