Add position sticky main thread scrolling reason [spv2]

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 340 matching lines @@
         bool oldStyleIsSticky = oldStyle && oldStyle->position() == StickyPosition;
 
         if (newStyleIsSticky != oldStyleIsSticky) {
             if (newStyleIsSticky) {
                 frameView->addStickyPositionObject();
                 // During compositing inputs update we'll have the scroll
                 // ancestor without having to walk up the tree and can compute
                 // the sticky position constraints then.
                 if (layer())
                     layer()->setNeedsCompositingInputsUpdate();
+
+                // TODO(pdr): When slimming paint v2 is enabled, we will need to
+                // invalidate the scroll paint property subtree for this so main
+                // thread scroll reasons are recomputed.
             } else {
                 // This may get re-added to viewport constrained objects if the object went
                 // from sticky to fixed.
                 frameView->removeViewportConstrainedObject(this);
                 frameView->removeStickyPositionObject();
 
                 // Remove sticky constraints for this layer.
                 if (layer()) {
                     DisableCompositingQueryAsserts disabler;
                     if (const PaintLayer* ancestorOverflowLayer = layer()->ancestorOverflowLayer())
                         ancestorOverflowLayer->getScrollableArea()->invalidateStickyConstraintsFor(layer());
                 }
+
+                // TODO(pdr): When slimming paint v2 is enabled, we will need to
+                // invalidate the scroll paint property subtree for this so main
+                // thread scroll reasons are recomputed.
             }
         }
 
         if (newStyleIsViewportConstained != oldStyleIsViewportConstrained) {
             if (newStyleIsViewportConstained && layer())
                 frameView->addViewportConstrainedObject(this);
             else
                 frameView->removeViewportConstrainedObject(this);
         }
     }
@@ skipping 800 matching lines @@
     if (rootElementStyle->hasBackground())
         return false;
 
     if (node() != document().firstBodyElement())
         return false;
 
     return true;
 }
 
 } // namespace blink