change the way windows are parented when their parent is set to NULL.

ui/aura_shell/shell.h

+// Copyright (c) 2011 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.
+
+#ifndef UI_AURA_SHELL_SHELL_H_
+#define UI_AURA_SHELL_SHELL_H_
+#pragma once
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "ui/aura/desktop_delegate.h"
+
+namespace aura {
+class Window;
+}
+
+namespace aura_shell {
+
+class Shell : public aura::DesktopDelegate {
+ public:
+  Shell();
+  virtual ~Shell();
+
+  void Init();
+
+ private:
+  aura::Window* GetContainer(int container_id);
+  const aura::Window* GetContainer(int container_id) const;
+
+  // Overridden from aura::DesktopDelegate:
+  virtual void AddChildToDefaultParent(aura::Window* window) OVERRIDE;
+  virtual aura::Window* GetTopmostWindowToActivate(
+      aura::Window* ignore) const OVERRIDE;
+
+  DISALLOW_COPY_AND_ASSIGN(Shell);
+};
+
+}  // namespace aura_shell
+
+#endif  // UI_AURA_SHELL_SHELL_H_