Scroll with Layers in views::ScrollView

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index 5b0fb57027e476c8eb01c62f718c3d1a2a639ce2..e4eae551de99ae23c5215c09f5179e8a8932e881 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -254,15 +254,6 @@ void SingleThreadProxy::DoCommit() {
             "461509 SingleThreadProxy::DoCommit7"));
     layer_tree_host_->FinishCommitOnImplThread(layer_tree_host_impl_.get());
 
-#if DCHECK_IS_ON()
-    // In the single-threaded case, the scale and scroll deltas should never be
-    // touched on the impl layer tree.

[Ian Vollick, 2016/08/12 13:57:42]

I presume that scroll to offset can now cause scroll deltas to be collected by
the LTHI making the first DCHECK fail, but we shouldn't be pinch-zooming, so the
page scale delta should still be 1.f, right?

If that's true, can you reinstate this code for the 2nd DCHECK, and add a
comment as to why we might get scroll deltas in the single thread proxy (since
it's counterintuitive).

[tapted, 2016/08/13 09:03:53]

Done (reinstated in DoBeginMainFrame() since the ProcessScrollDeltas() is needed
for the DCHECK).

-    std::unique_ptr<ScrollAndScaleSet> scroll_info =
-        layer_tree_host_impl_->ProcessScrollDeltas();
-    DCHECK(scroll_info->scrolls.empty());
-    DCHECK_EQ(1.f, scroll_info->page_scale_delta);
-#endif
-
     if (scheduler_on_impl_thread_)
       scheduler_on_impl_thread_->DidCommit();
 
@@ -802,6 +793,10 @@ void SingleThreadProxy::BeginMainFrame(const BeginFrameArgs& begin_frame_args) {
 
 void SingleThreadProxy::DoBeginMainFrame(
     const BeginFrameArgs& begin_frame_args) {
+  std::unique_ptr<ScrollAndScaleSet> scroll_info =
+      layer_tree_host_impl_->ProcessScrollDeltas();
+  layer_tree_host_->ApplyScrollAndScale(scroll_info.get());

[Ian Vollick, 2016/08/12 13:57:42]

Again, please add a comment as this is counterintuitive.

[tapted, 2016/08/13 09:03:53]

Done.

+
   layer_tree_host_->WillBeginMainFrame();
   layer_tree_host_->BeginMainFrame(begin_frame_args);
   layer_tree_host_->AnimateLayers(begin_frame_args.frame_time);