Fix incorrect use of coords conversion for sticky elements

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index c438cf02eb89b27ee507e657c12a2d6998906d3c..c4210b99139b9e7b071bcc5690ddbb2513571847 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -317,8 +317,16 @@ void CompositedLayerMapping::UpdateStickyConstraints(
     // 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 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->ConvertToLayerCoords(ancestor_overflow_layer,
+                                                  enclosing_layer_offset);
+    }
+
     DCHECK(!ScrollParent() || ScrollParent() == ancestor_overflow_layer);
     if (!ScrollParent() && compositing_container != ancestor_overflow_layer) {
       enclosing_layer_offset += LayoutSize(
@@ -338,7 +346,9 @@ void CompositedLayerMapping::UpdateStickyConstraints(
         FloatPoint(-enclosing_layer_offset) -
         FloatSize(ContentOffsetInCompositingLayer()));
 
-    if (compositing_container != ancestor_overflow_layer) {
+    // When we have a scroll parent, the content offset is computed by
+    // source_offset on the transform node.
+    if (!ScrollParent() && compositing_container != ancestor_overflow_layer) {
       parent_relative_sticky_box_offset.MoveBy(
           FloatPoint(compositing_container->GetCompositedLayerMapping()
                          ->ContentOffsetInCompositingLayer()));