Always disable "content has shadow".

ui/base/cocoa/underlay_opengl_hosting_window.mm

 // 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.
 
 #import "ui/base/cocoa/underlay_opengl_hosting_window.h"
 
 #import <objc/runtime.h>
 
 #include "base/logging.h"
 #include "base/mac/mac_util.h"
@@ skipping 90 matching lines @@
   // most Chromium windows, not because this is related to its functionality.)
   DCHECK(!NSIsEmptyRect(contentRect));
   if ((self = [super initWithContentRect:contentRect
                                styleMask:windowStyle
                                  backing:bufferingType
                                    defer:deferCreation])) {
     // OpenGL-accelerated content works by punching holes in windows. Therefore
     // all windows hosting OpenGL content must not be opaque.
     [self setOpaque:NO];
 
+    // Always disable "content has shadow". This class assumes that all holes
+    // punched in the window are intentional, in order to show accelerated
+    // content underneath, so those holes cannot be allowed to cause holes in
+    // the shadow. Note that the edges trimmed from around the periphery of a
+    // window (through use of transparency) don't count as content area with
+    // regards to "content has shadow".
+    [self _setContentHasShadow:NO];

[Nico, 2012/11/27 00:28:12]

Hum, I would've expected this to force a regulare rectangular window shadow for
the bubbles, instead of a shadow that follows the bubble shape. But if it looks
good on 10.6-10.8, lgtm

[Avi (use Gerrit), 2012/11/27 14:51:53]

That's the magic of "content has shadow = NO"; the shadow region is
scanline-by-scanline, first opaque pixel to last opaque pixel.

Or at least I hope so. I also need to verify on HiDPI too...

+
+    // Only set up opaques if the window is a proper window with a title bar
+    // and all.
     if (windowStyle & NSTitledWindowMask) {
-      // Only fiddle with shadows if the window is a proper window with a
-      // title bar and all.
-      [self _setContentHasShadow:NO];
 
       NSView* rootView = [[self contentView] superview];
       const NSRect rootBounds = [rootView bounds];
 
       // On 10.7/8, the bottom corners of the window are rounded by magic at a
       // deeper level than the NSThemeFrame, so it is OK to have the opaques
       // go all the way to the bottom.
       const CGFloat kTopEdgeInset = 16;
       const CGFloat kAlphaValueJustOpaqueEnough = 0.005;
 
@@ skipping 12 matching lines @@
                                        NSViewHeightSizable];
       [rightOpaque setAlphaValue:kAlphaValueJustOpaqueEnough];
       [rootView addSubview:rightOpaque];
     }
   }
 
   return self;
 }
 
 @end