[RootLayerScrolls] Annotate non-fast-scroll regions to correct layer

third_party/WebKit/Source/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All
  * Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 162 matching lines @@
 }
 
 ClientRectList* Page::nonFastScrollableRects(const LocalFrame* frame) {
   if (ScrollingCoordinator* scrollingCoordinator =
           this->scrollingCoordinator()) {
     // Hits in compositing/iframes/iframe-composited-scrolling.html
     DisableCompositingQueryAsserts disabler;
     scrollingCoordinator->updateAfterCompositingChangeIfNeeded();
   }
 
-  if (!frame->view()->layerForScrolling())
-    return ClientRectList::create();
-
-  // Now retain non-fast scrollable regions
-  return ClientRectList::create(frame->view()
-                                    ->layerForScrolling()
-                                    ->platformLayer()
-                                    ->nonFastScrollableRegion());
+  return frame->view()
+      ->layoutViewportScrollableArea()
+      ->nonFastScrollableRects();

[skobes, 2017/01/27 20:14:18]

Could this just read
layoutViewportScrollableArea()->layerForScrolling()->platformLayer()->nonFastScrollableRegion()?

That would match what ScrollingCoordinator already does on the write path, and
avoid the need for nonFastScrollableRects plumbing in ScrollableArea, FrameView,
PLSA, and PaintLayer.

(The plumbing not only adds complexity but is actively misleading because it
makes you think we support non-fast-scrollable regions with per-scroller
granularity, when in fact they will return an empty ClientRectList for anything
other than the main frame's layout viewport.)

[szager1, 2017/01/27 22:32:49]

layerForScrolling()
 layer()->compositedLayerMapping()
   DCHECK(isAllowedToQueryCompositingState())
BOOM!

All of the new plumbing is just to avoid that DCHECK.  I'm open to any
reasonable solution.  Maybe compositedLayerMappingForTesting?  I dunno.

 }
 
 void Page::setMainFrame(Frame* mainFrame) {
   // Should only be called during initialization or swaps between local and
   // remote frames.
   // FIXME: Unfortunately we can't assert on this at the moment, because this
   // is called in the base constructor for both LocalFrame and RemoteFrame,
   // when the vtables for the derived classes have not yet been setup.
   m_mainFrame = mainFrame;
 }
@@ skipping 342 matching lines @@
     : chromeClient(nullptr),
       contextMenuClient(nullptr),
       editorClient(nullptr),
       spellCheckerClient(nullptr) {}
 
 Page::PageClients::~PageClients() {}
 
 template class CORE_TEMPLATE_EXPORT Supplement<Page>;
 
 }  // namespace blink