Mac: Don't repaint scrollbars every frame

cc/layers/painted_scrollbar_layer_impl.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_impl.h"
 
 #include <algorithm>
 
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/layers/layer.h"
@@ skipping 14 matching lines @@
       new PaintedScrollbarLayerImpl(tree_impl, id, orientation));
 }
 
 PaintedScrollbarLayerImpl::PaintedScrollbarLayerImpl(
     LayerTreeImpl* tree_impl,
     int id,
     ScrollbarOrientation orientation)
     : ScrollbarLayerImplBase(tree_impl, id, orientation, false, false),
       track_ui_resource_id_(0),
       thumb_ui_resource_id_(0),
+      thumb_opacity_(1.f),
       internal_contents_scale_(1.f),
       thumb_thickness_(0),
       thumb_length_(0),
       track_start_(0),
-      track_length_(0) {
-}
+      track_length_(0) {}
 
 PaintedScrollbarLayerImpl::~PaintedScrollbarLayerImpl() {}
 
 scoped_ptr<LayerImpl> PaintedScrollbarLayerImpl::CreateLayerImpl(
     LayerTreeImpl* tree_impl) {
   return PaintedScrollbarLayerImpl::Create(tree_impl, id(), orientation());
 }
 
 void PaintedScrollbarLayerImpl::PushPropertiesTo(LayerImpl* layer) {
   ScrollbarLayerImplBase::PushPropertiesTo(layer);
 
   PaintedScrollbarLayerImpl* scrollbar_layer =
       static_cast<PaintedScrollbarLayerImpl*>(layer);
 
   scrollbar_layer->set_internal_contents_scale_and_bounds(
       internal_contents_scale_, internal_content_bounds_);
 
   scrollbar_layer->SetThumbThickness(thumb_thickness_);
   scrollbar_layer->SetThumbLength(thumb_length_);
   scrollbar_layer->SetTrackStart(track_start_);
   scrollbar_layer->SetTrackLength(track_length_);
 
   scrollbar_layer->set_track_ui_resource_id(track_ui_resource_id_);
   scrollbar_layer->set_thumb_ui_resource_id(thumb_ui_resource_id_);
+
+  scrollbar_layer->set_thumb_opacity(thumb_opacity_);
 }
 
 bool PaintedScrollbarLayerImpl::WillDraw(DrawMode draw_mode,
                                   ResourceProvider* resource_provider) {
   DCHECK(draw_mode != DRAW_MODE_RESOURCELESS_SOFTWARE);
   return LayerImpl::WillDraw(draw_mode, resource_provider);
 }
 
 void PaintedScrollbarLayerImpl::AppendQuads(
     RenderPass* render_pass,
@@ skipping 20 matching lines @@
   gfx::Rect scaled_visible_thumb_quad_rect = gfx::ScaleToEnclosingRect(
       visible_thumb_quad_rect, internal_contents_scale_);
 
   ResourceId thumb_resource_id =
       layer_tree_impl()->ResourceIdForUIResource(thumb_ui_resource_id_);
   ResourceId track_resource_id =
       layer_tree_impl()->ResourceIdForUIResource(track_ui_resource_id_);
 
   if (thumb_resource_id && !visible_thumb_quad_rect.IsEmpty()) {
     gfx::Rect opaque_rect;
-    const float opacity[] = {1.0f, 1.0f, 1.0f, 1.0f};
+    const float opacity[] = {thumb_opacity_, thumb_opacity_, thumb_opacity_,
+                             thumb_opacity_};
     TextureDrawQuad* quad =
         render_pass->CreateAndAppendDrawQuad<TextureDrawQuad>();
     quad->SetNew(shared_quad_state, scaled_thumb_quad_rect, opaque_rect,
                  scaled_visible_thumb_quad_rect, thumb_resource_id,
                  premultipled_alpha, uv_top_left, uv_bottom_right,
                  SK_ColorTRANSPARENT, opacity, flipped, nearest_neighbor);
     ValidateQuadResources(quad);
   }
 
   gfx::Rect track_quad_rect(bounds());
@@ skipping 70 matching lines @@
 
 bool PaintedScrollbarLayerImpl::IsThumbResizable() const {
   return false;
 }
 
 const char* PaintedScrollbarLayerImpl::LayerTypeAsString() const {
   return "cc::PaintedScrollbarLayerImpl";
 }
 
 }  // namespace cc