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

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

Index: third_party/WebKit/Source/core/page/Page.cpp
diff --git a/third_party/WebKit/Source/core/page/Page.cpp b/third_party/WebKit/Source/core/page/Page.cpp
index 6557e08891bf42e14e4b9151a32a90e3ea62c8ad..c9ca14fd86d5794fedf0e55e3b2ca23321c61b0d 100644
--- a/third_party/WebKit/Source/core/page/Page.cpp
+++ b/third_party/WebKit/Source/core/page/Page.cpp
@@ -180,14 +180,9 @@ ClientRectList* Page::nonFastScrollableRects(const LocalFrame* frame) {
     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) {