Reduce overdraw on bookmark bar

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

Index: chrome/browser/ui/views/bookmarks/bookmark_bar_view.cc
diff --git a/chrome/browser/ui/views/bookmarks/bookmark_bar_view.cc b/chrome/browser/ui/views/bookmarks/bookmark_bar_view.cc
index ce37e63e3dbf78cae113307a1998cf5fb894aa80..c3e62f45dc1f17229926dff5097266ea1bba6122 100644
--- a/chrome/browser/ui/views/bookmarks/bookmark_bar_view.cc
+++ b/chrome/browser/ui/views/bookmarks/bookmark_bar_view.cc
@@ -601,12 +601,6 @@ BookmarkBarView::BookmarkBarView(Browser* browser, BrowserView* browser_view)
       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);
-
   size_animation_.Reset(1);
 }
 
@@ -653,12 +647,19 @@ void BookmarkBarView::SetBookmarkBarState(
     BookmarkBar::AnimateChangeType animate_type) {
   if (animate_type == BookmarkBar::ANIMATE_STATE_CHANGE &&
       animations_enabled) {
+    if (!layer()) {
+      SetPaintToLayer();
+      layer()->SetMasksToBounds(true);
+      layer()->SetFillsBoundsOpaquely(false);
+    }
     animating_detached_ = (state == BookmarkBar::DETACHED ||
                            bookmark_bar_state_ == BookmarkBar::DETACHED);
     if (state == BookmarkBar::SHOW)
       size_animation_.Show();
     else
       size_animation_.Hide();
+    if (layer())
+      DestroyLayer();

[Evan Stade, 2017/05/24 17:34:30]

I am surprised this works. You're destroying the layer almost as soon as it's
being created. I would have thought you'd need to wait until AnimationEnded to
destroy the layer.

(also, I think it's safe to call DestroyLayer even if you don't have a layer.)

[sky, 2017/05/24 21:32:05]

I think it would be better to key off whether any descendant views have layers.
You can do this via OnChildLayerChanged(). I'm suggesting any time that function
is called you check if any visible descendants have layers. If they don't, then
the bookmarkbar shouldn't need a layer.

   } else {
     size_animation_.Reset(state == BookmarkBar::SHOW ? 1 : 0);
   }