Correct for enclosing layers scroll position in sticky_data->main_thread_offset

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
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 290 matching lines @@
     }
   }
 
   WebLayerStickyPositionConstraint webConstraint;
   if (sticky) {
     const StickyPositionScrollingConstraints& constraints =
         ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap().get(
             &m_owningLayer);
 
     // Find the layout offset of the unshifted sticky box within its enclosing
-    // layer.
+    // layer. If the enclosing layer is not the scroller, the offset must be
+    // adjusted to include the scroll offset to keep it relative.
+    PaintLayer* enclosingLayer =
+        m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf);
     LayoutPoint enclosingLayerOffset;
-    m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf)
-        ->convertToLayerCoords(m_owningLayer.ancestorOverflowLayer(),
-                               enclosingLayerOffset);
+    enclosingLayer->convertToLayerCoords(m_owningLayer.ancestorOverflowLayer(),
+                                         enclosingLayerOffset);
+    if (enclosingLayer != m_owningLayer.ancestorOverflowLayer() &&
+        enclosingLayer->containingLayer()) {
+      enclosingLayerOffset += enclosingLayer->containingLayer()
+                                  ->layoutBox()
+                                  ->scrolledContentOffset();

[flackr, 2017/02/22 14:23:09]

scrolledContentOffset requires that the box has an overflow clip but what if the
containing layer is not a scroller? Is this required if the enclosing layer is a
descendant of the scroller (in which case we can use ancestorOverflowLayer's
scroll offset) or just a child?

[smcgruer, 2017/02/22 15:42:30]

Done.

+    }
+
     FloatPoint stickyBoxOffset =
         constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);
     stickyBoxOffset.moveBy(FloatPoint(-enclosingLayerOffset) -
                            FloatSize(contentOffsetInCompositingLayer()));
 
     webConstraint.isSticky = true;
     webConstraint.isAnchoredLeft =
         constraints.anchorEdges() &
         StickyPositionScrollingConstraints::AnchorEdgeLeft;
@@ skipping 3152 matching lines @@
   } else if (graphicsLayer == m_decorationOutlineLayer.get()) {
     name = "Decoration Layer";
   } else {
     ASSERT_NOT_REACHED();
   }
 
   return name;
 }
 
 }  // namespace blink