Fix incorrect use of coords conversion for sticky elements

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 299 matching lines @@
     //
     // Starting from the scroll container relative location, removing the
     // enclosing layer's offset and the content offset in the composited layer
     // results in the parent-layer relative offset.
     FloatPoint parent_relative_sticky_box_offset =
         constraints.ScrollContainerRelativeStickyBoxRect().Location();
 
     // The enclosing layers offset returned from |convertToLayerCoords| must be
     // adjusted for both scroll and ancestor sticky elements.
     LayoutPoint enclosing_layer_offset;
-    compositing_container->ConvertToLayerCoords(ancestor_overflow_layer,
-                                                enclosing_layer_offset);
+    // The offset from the compositing_container to the ancestor_overflow_layer
+    // when we have a scroll parent or a fix positioned container is computed by
+    // the source_to_parent vector on the transform node.
+    // TODO(yigu): Eventually we want to remove the dependancy of
+    // source_to_parent from StickyPositionOffset in property_tree.cc.
+    if (!ScrollParent() ||
+        !compositing_container->GetLayoutObject().IsFixedPositioned()) {

[flackr, 2017/05/19 18:28:31]

I thought we wanted to only compute this offset if the compositing_container was
a descendant but this will compute it even if the compositing container is an
ancestor but is not fixed position (i.e. the scroll parent = true case).

[yigu, 2017/05/19 19:50:45]

Done.

+      compositing_container->ConvertToLayerCoords(ancestor_overflow_layer,
+                                                  enclosing_layer_offset);
+    }
+
     DCHECK(!ScrollParent() || ScrollParent() == ancestor_overflow_layer);
     if (!ScrollParent() && compositing_container != ancestor_overflow_layer) {
       enclosing_layer_offset += LayoutSize(
           ancestor_overflow_layer->GetScrollableArea()->GetScrollOffset());
     }
     // TODO(smcgruer): Until http://crbug.com/702229 is fixed, the nearest
     // sticky ancestor may be non-composited which will make this offset wrong.
     if (const LayoutBoxModelObject* ancestor =
             constraints.NearestStickyAncestor()) {
       enclosing_layer_offset -=
@@ skipping 3330 matching lines @@
   } else if (graphics_layer == decoration_outline_layer_.get()) {
     name = "Decoration Layer";
   } else {
     NOTREACHED();
   }
 
   return name;
 }
 
 }  // namespace blink