Mac: Don't repaint scrollbars every frame

cc/layers/painted_scrollbar_layer.cc

 // Copyright 2013 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 "cc/layers/painted_scrollbar_layer.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "base/basictypes.h"
@@ skipping 31 matching lines @@
 PaintedScrollbarLayer::PaintedScrollbarLayer(const LayerSettings& settings,
                                              scoped_ptr<Scrollbar> scrollbar,
                                              int scroll_layer_id)
     : Layer(settings),
       scrollbar_(std::move(scrollbar)),
       scroll_layer_id_(scroll_layer_id),
       internal_contents_scale_(1.f),
       thumb_thickness_(scrollbar_->ThumbThickness()),
       thumb_length_(scrollbar_->ThumbLength()),
       is_overlay_(scrollbar_->IsOverlay()),
-      has_thumb_(scrollbar_->HasThumb()) {
+      has_thumb_(scrollbar_->HasThumb()),
+      thumb_opacity_(scrollbar_->ThumbOpacity()) {
   if (!scrollbar_->IsOverlay())
     SetShouldScrollOnMainThread(true);
 }
 
 PaintedScrollbarLayer::~PaintedScrollbarLayer() {}
 
 int PaintedScrollbarLayer::ScrollLayerId() const {
   return scroll_layer_id_;
 }
 
@@ skipping 174 matching lines @@
 
   if (!has_thumb_ && thumb_resource_) {
     thumb_resource_ = nullptr;
     SetNeedsPushProperties();
     updated = true;
   }
 
   if (update_rect_.IsEmpty() && track_resource_)
     return updated;
 
-  track_resource_ = ScopedUIResource::Create(
-      layer_tree_host(),
-      RasterizeScrollbarPart(track_layer_rect, scaled_track_rect, TRACK));
+  if (!track_resource_ || scrollbar_->NeedsPaintPart(TRACK)) {
+    track_resource_ = ScopedUIResource::Create(
+        layer_tree_host(),
+        RasterizeScrollbarPart(track_layer_rect, scaled_track_rect, TRACK));
+  }
 
   gfx::Rect thumb_layer_rect = OriginThumbRect();
   gfx::Rect scaled_thumb_rect =
       ScrollbarLayerRectToContentRect(thumb_layer_rect);
   if (has_thumb_ && !scaled_thumb_rect.IsEmpty()) {
-    thumb_resource_ = ScopedUIResource::Create(
-        layer_tree_host(),
-        RasterizeScrollbarPart(thumb_layer_rect, scaled_thumb_rect, THUMB));
+    if (!thumb_resource_ || scrollbar_->NeedsPaintPart(THUMB)) {
+      thumb_resource_ = ScopedUIResource::Create(
+          layer_tree_host(),
+          RasterizeScrollbarPart(thumb_layer_rect, scaled_thumb_rect, THUMB));
+    }
+    thumb_opacity_ = scrollbar_->ThumbOpacity();
   }
 
   // UI resources changed so push properties is needed.
   SetNeedsPushProperties();
   updated = true;
   return updated;
 }
 
 UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(
     const gfx::Rect& layer_rect,
@@ skipping 25 matching lines @@
 
   scrollbar_->PaintPart(&skcanvas, part, layer_rect);
   // Make sure that the pixels are no longer mutable to unavoid unnecessary
   // allocation and copying.
   skbitmap.setImmutable();
 
   return UIResourceBitmap(skbitmap);
 }
 
 }  // namespace cc