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..ac7a882cf16d9f23d8cd8dab4921d29c19da833a 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -317,8 +317,17 @@ 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 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(