Defer scroll status determination until scrolling has occurred

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1751 matching lines @@
 
 LayerImpl* LayerTreeHostImpl::OuterViewportScrollLayer() const {
   return active_tree_->OuterViewportScrollLayer();
 }
 
 LayerImpl* LayerTreeHostImpl::CurrentlyScrollingLayer() const {
   return active_tree_->CurrentlyScrollingLayer();
 }
 
 bool LayerTreeHostImpl::IsCurrentlyScrolling() const {
-  return CurrentlyScrollingLayer() ||
+  return (did_lock_scrolling_layer_ && CurrentlyScrollingLayer()) ||
          (InnerViewportScrollLayer() &&
           InnerViewportScrollLayer()->IsExternalFlingActive()) ||
          (OuterViewportScrollLayer() &&
           OuterViewportScrollLayer()->IsExternalFlingActive());
 }
 
 // Content layers can be either directly scrollable or contained in an outer
 // scrolling layer which applies the scroll transform. Given a content layer,
 // this function returns the associated scroll layer if any.
 static LayerImpl* FindScrollLayerForContentLayer(LayerImpl* layer_impl) {
@@ skipping 618 matching lines @@
 
   if (scroll_on_main_thread) {
     UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
     return ScrollOnMainThread;
   }
 
   if (scrolling_layer_impl) {
     active_tree_->SetCurrentlyScrollingLayer(scrolling_layer_impl);
     should_bubble_scrolls_ = (type != NonBubblingGesture);
     wheel_scrolling_ = (type == Wheel);
-    client_->RenewTreePriority();

[brianderson, 2014/10/23 20:55:51]

Why is this removed?

Is it because we'll only have a scrolling layer at this point, but not have
actually scrolled?

[jdduke (slow), 2014/10/23 22:12:49]

Right, unless there's another side effect of this method that might cause a
different renewal priority? I guess it couldn't hurt to keep it? I'll add it
back.

     UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
     return ScrollStarted;
   }
   return ScrollIgnored;
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
     const gfx::Point& viewport_point,
     const gfx::Vector2dF& scroll_delta) {
   if (LayerImpl* layer_impl = CurrentlyScrollingLayer()) {
@@ skipping 1055 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc