Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/wm/workspace/workspace_layout_manager_backdrop.h

+// Copyright (c) 2014 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 ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_BACKDROP_H_
+#define ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_BACKDROP_H_
+
+#include "ui/aura/window_observer.h"
+
+namespace aura {
+class Window;
+}
+
+namespace ui {
+class Layer;
+}
+
+namespace views {
+class Widget;
+}
+
+namespace ash {
+
+namespace internal {
+
+// A background which gets created for a container |window| and which gets
+// stacked behind the topmost window (within that container) covering the
+// entire container.
+class WorkspaceLayoutManagerBackdrop : public aura::WindowObserver {
+ public:
+  explicit WorkspaceLayoutManagerBackdrop(aura::Window* container);
+  virtual ~WorkspaceLayoutManagerBackdrop();
+
+  void RestackOrHideWindow();
+
+  // WindowObserver overrides:
+  virtual void OnWindowBoundsChanged(aura::Window* window,
+                                     const gfx::Rect& old_bounds,
+                                     const gfx::Rect& new_bounds) OVERRIDE;
+
+ private:
+  // Position & size the background over the container window.
+  void AdjustToContainerBounds();
+
+  // Show the overlay.
+  void Show();
+
+  // The background which covers the rest of the screen.
+  views::Widget* background_;
+
+  // The window which is being "maximized".
+  aura::Window* container_;
+
+  // If true, the |RestackOrHideWindow| might recurse.
+  bool inside_;
+
+  DISALLOW_COPY_AND_ASSIGN(WorkspaceLayoutManagerBackdrop);
+};
+
+}  // namespace internal
+
+}  // namespace ash
+
+#endif  // ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_BACKDROP_H_