Do not promote position sticky or fixed elements unless they move with scroll.

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 231 matching lines @@
   ImageQualityController::remove(*this);
 
   // A continuation of this LayoutObject should be destroyed at subclasses.
   ASSERT(!continuation());
 
   if (isPositioned()) {
     // Don't use this->view() because the document's layoutView has been set to
     // 0 during destruction.
     if (LocalFrame* frame = this->frame()) {
       if (FrameView* frameView = frame->view()) {
-        if (style()->hasViewportConstrainedPosition())
+        if (style()->hasViewportConstrainedPosition() ||
+            style()->hasStickyConstrainedPosition())

[chrishtr, 2017/03/16 00:24:03]

This was a bug before?

[flackr, 2017/03/20 21:04:59]

Not so much a bug, just hasStickyConstrainedPosition now also checks that it is
actually constrained by one of top/bottom/left/right. If it's not we should
treat position sticky as if it is position relative (i.e. not cause slow
scrolling, not promote it, etc). I realized this also needs to be updated in
CompositingInputsUpdater, so I've updated it there and added some tests.
Interestingly, as far as I can tell the SPv2 version never adds sticky position
to the frameview viewport constrained set (but with root layer scrolling we
don't need this anymore).

           frameView->removeViewportConstrainedObject(*this);
       }
     }
   }
 
   LayoutObject::willBeDestroyed();
 
   destroyLayer();
 }
 
@@ skipping 1204 matching lines @@
   if (rootElementStyle->hasBackground())
     return false;
 
   if (node() != document().firstBodyElement())
     return false;
 
   return true;
 }
 
 }  // namespace blink