Mark elements as viewport constrained when they become sticky positioned

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 381 matching lines @@
         oldStyle && oldStyle->position() == EPosition::kSticky;
 
     if (newStyleIsSticky != oldStyleIsSticky) {
       if (newStyleIsSticky) {
         // 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();
 
+        frameView->addViewportConstrainedObject(this);

[flackr, 2017/02/21 00:38:14]

There's a call to add sticky position items to the set of viewport constrained
objects which happens during the compositing inputs update. Can you check why
the sticky element isn't getting added then? Doing it here is incorrect as we
don't know whether it is contained by an overflow scroller yet.

[smcgruer, 2017/02/21 15:50:05]

Done.

+
         // 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);
 
         // Remove sticky constraints for this layer.
         if (layer()) {
@@ skipping 955 matching lines @@
   if (rootElementStyle->hasBackground())
     return false;
 
   if (node() != document().firstBodyElement())
     return false;
 
   return true;
 }
 
 }  // namespace blink