Proactive tweaking of window shadow code to try to fix problem.

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 102 matching lines @@
 
       if (windowStyle & NSTitledWindowMask) {
         // Only fiddle with shadows if the window is a proper window with a
         // title bar and all. (The invisible opaque area technique only works on
         // > 10.5, but that is guaranteed by this point.)
         [self _setContentHasShadow:NO];
 
         NSView* rootView = [[self contentView] superview];
         const NSRect rootBounds = [rootView bounds];
 
-        const CGFloat kEdgeInset = 16;
-        const CGFloat kAlphaValueJustOpaqueEnough = 0.002;
+        // 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 = 0.003;

[Nico, 2012/06/01 17:07:23]

Why the alpha value tweak?

 
         scoped_nsobject<NSView> leftOpaque([[OpaqueView alloc] initWithFrame:
-            NSMakeRect(NSMinX(rootBounds), NSMinY(rootBounds) + kEdgeInset,
-                       1, NSHeight(rootBounds) - 2 * kEdgeInset)]);
+            NSMakeRect(NSMinX(rootBounds), NSMinY(rootBounds),
+                       1, NSHeight(rootBounds) - kTopEdgeInset)]);
         [leftOpaque setAutoresizingMask:NSViewMaxXMargin |
                                         NSViewHeightSizable];
         [leftOpaque setAlphaValue:kAlphaValueJustOpaqueEnough];
         [rootView addSubview:leftOpaque];
 
         scoped_nsobject<NSView> rightOpaque([[OpaqueView alloc] initWithFrame:
-            NSMakeRect(NSMaxX(rootBounds) - 1, NSMinY(rootBounds) + kEdgeInset,
-                       1, NSHeight(rootBounds) - 2 * kEdgeInset)]);
+            NSMakeRect(NSMaxX(rootBounds) - 1, NSMinY(rootBounds),
+                       1, NSHeight(rootBounds) - kTopEdgeInset)]);
         [rightOpaque setAutoresizingMask:NSViewMinXMargin |
                                          NSViewHeightSizable];
         [rightOpaque setAlphaValue:kAlphaValueJustOpaqueEnough];
         [rootView addSubview:rightOpaque];
       }
     }
   }
 
   return self;
 }
 
 @end