[cc] Change semantics of fixed-pos container layer

third_party/WebKit/Source/core/frame/VisualViewport.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 329 matching lines @@
 
   // FIXME: The root transform layer should only be created on demand.
   root_transform_layer_ = GraphicsLayer::Create(this);
   inner_viewport_container_layer_ = GraphicsLayer::Create(this);
   overscroll_elasticity_layer_ = GraphicsLayer::Create(this);
   page_scale_layer_ = GraphicsLayer::Create(this);
   inner_viewport_scroll_layer_ = GraphicsLayer::Create(this);
   overlay_scrollbar_horizontal_ = GraphicsLayer::Create(this);
   overlay_scrollbar_vertical_ = GraphicsLayer::Create(this);
 
+  // What is it really for? Are there any fixed-pos layers positioned
+  // by the VisualViewport actually?

[bokan, 2017/04/26 21:17:56]

I believe this was a temporary state meant only to support running in either
modes that never got removed. IMO, we can probably get rid of this, I think only
the outer viewport container layer (the PLC's container layer) should be a
ContainerForFixedPositionLayers.

[trchen, 2017/05/01 21:24:13]

I removed it in patch set 2, but on a second thought, I think we'd better change
on thing at a time so added it back.

   ScrollingCoordinator* coordinator = GetPage().GetScrollingCoordinator();
   DCHECK(coordinator);
   coordinator->SetLayerIsContainerForFixedPositionLayers(
-      inner_viewport_scroll_layer_.get(), true);
+      page_scale_layer_.get(), true);
 
   // Set masks to bounds so the compositor doesn't clobber a manually
   // set inner viewport container layer size.
   inner_viewport_container_layer_->SetMasksToBounds(
       GetPage().GetSettings().GetMainFrameClipsContent());
   inner_viewport_container_layer_->SetSize(FloatSize(size_));
 
   inner_viewport_scroll_layer_->PlatformLayer()->SetScrollClipLayer(
       inner_viewport_container_layer_->PlatformLayer());
   inner_viewport_scroll_layer_->PlatformLayer()->SetUserScrollable(true, true);
@@ skipping 488 matching lines @@
   } else if (graphics_layer == root_transform_layer_.get()) {
     name = "Root Transform Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink