Fix position:sticky location bug when scroller is not a stacking context

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
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@@ skipping 301 matching lines @@
         ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap().at(
             &m_owningLayer);
 
     // Find the layout offset of the unshifted sticky box within its
     // compositingContainer. If the enclosing layer is not the scroller, then
     // the offset must be adjusted to include the scroll offset to keep it
     // relative to compositingContainer.
     LayoutPoint enclosingLayerOffset;
     compositingContainer->convertToLayerCoords(ancestorOverflowLayer,
                                                enclosingLayerOffset);
-    if (compositingContainer != ancestorOverflowLayer) {
+    if (!scrollParent() && compositingContainer != ancestorOverflowLayer) {

[flackr, 2017/03/23 16:58:03]

It's probably worth "documenting" why we do this with a DCHECK to show that if
we have a scroll parent it must be our ancestor overflow layer:
DCHECK(!scrollParent() || scrollParent() == ancestorOverflowLayer);

[smcgruer, 2017/03/23 17:22:48]

Done.

       enclosingLayerOffset += LayoutSize(
           ancestorOverflowLayer->getScrollableArea()->getScrollOffset());
     }
 
     FloatPoint stickyBoxOffset =
         constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);
     stickyBoxOffset.moveBy(FloatPoint(-enclosingLayerOffset) -
                            FloatSize(contentOffsetInCompositingLayer()));
 
@@ skipping 3165 matching lines @@
   } else if (graphicsLayer == m_decorationOutlineLayer.get()) {
     name = "Decoration Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink