Reduce overdraw on bookmark bar

chrome/browser/ui/views/bookmarks/bookmark_bar_view.cc

 // 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.
 
 #include "chrome/browser/ui/views/bookmarks/bookmark_bar_view.h"
 
 #include <stdint.h>
 
 #include <algorithm>
 #include <limits>
@@ skipping 587 matching lines @@
       throbbing_view_(nullptr),
       bookmark_bar_state_(BookmarkBar::SHOW),
       animating_detached_(false),
       show_folder_method_factory_(this) {
   set_id(VIEW_ID_BOOKMARK_BAR);
   Init();
 
   // Don't let the bookmarks show on top of the location bar while animating.
   SetPaintToLayer();
   layer()->SetMasksToBounds(true);
-  layer()->SetFillsBoundsOpaquely(false);
+  layer()->SetFillsBoundsOpaquely(true);

[sky, 2017/05/26 13:40:53]

I'm surprised this would work. Don't we need the the bookmark bar to show
through in some cases? Same themes or animations or on the ntp when detached?

[yiyix, 2017/05/26 18:30:33]

I believe that it's opaque for both attached and detached bookmark bars. 
For themes:
For attached case, we first paint background color of the toolbar, then we paint
the image from the theme for the bookmark bar. (as shown in
https://cs.chromium.org/chromium/src/chrome/browser/ui/views/frame/browser_vi...
). I believe the background of the toolbar is always opaque. I found this bug,
which requests it to be transparent and marked as "won't fix",
https://bugs.chromium.org/p/chromium/issues/detail?id=244892. I also tried to
search transparent theme in the app store and could not find any. So I was
convinced it's safe to mark the layer as opaque.

For detached case, I found this comment, which explains the color for view must
be painted opaquely and the layer can be semi-transparent, so it's safe to make
the bookmark bar layer as opaque as well.
https://cs.chromium.org/chromium/src/chrome/browser/ui/views/frame/browser_vi...

 
   size_animation_.Reset(1);
 }
 
 BookmarkBarView::~BookmarkBarView() {
   if (model_)
     model_->RemoveObserver(this);
 
   // It's possible for the menu to outlive us, reset the observer to make sure
   // it doesn't have a reference to us.
@@ skipping 1546 matching lines @@
     return;
   apps_page_shortcut_->SetVisible(visible);
   UpdateBookmarksSeparatorVisibility();
   LayoutAndPaint();
 }
 
 void BookmarkBarView::OnShowManagedBookmarksPrefChanged() {
   if (UpdateOtherAndManagedButtonsVisibility())
     LayoutAndPaint();
 }