RenderWidget popup should be a NSWindow so it can go outside the main window.

content/browser/renderer_host/render_widget_host_view_mac.h

 // 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.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_MAC_H_
 #define CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_MAC_H_
 
 #import <Cocoa/Cocoa.h>
 #include <list>
 #include <map>
@@ skipping 459 matching lines @@
 
   // The text to be shown in the tooltip, supplied by the renderer.
   string16 tooltip_text_;
 
   // Factory used to safely scope delayed calls to ShutdownHost().
   base::WeakPtrFactory<RenderWidgetHostViewMac> weak_factory_;
 
   // selected text on the renderer.
   std::string selected_text_;
 
+  // The window used for popup widgets.
+  scoped_nsobject<NSWindow> popup_window_;
+
   // The fullscreen window used for pepper flash.
   scoped_nsobject<NSWindow> pepper_fullscreen_window_;
   scoped_nsobject<FullscreenWindowManager> fullscreen_window_manager_;
   // Our parent host view, if this is fullscreen.  NULL otherwise.
   RenderWidgetHostViewMac* fullscreen_parent_host_view_;
 
   // List of pending swaps for deferred acking:
   //   pairs of (route_id, gpu_host_id).
   std::list<std::pair<int32, int32> > pending_swap_buffers_acks_;
 
   // The current composition character range and its bounds.
   ui::Range composition_range_;
   std::vector<gfx::Rect> composition_bounds_;
 
   // The current caret bounds.
   gfx::Rect caret_rect_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostViewMac);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_MAC_H_