Handle nested position:sticky elements correctly (main thread)

third_party/WebKit/Source/core/layout/LayoutObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.cpp b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
index ac03b02a63fada3e05335e07899878a16129e44e..72497e2872c9b1e0c09d35061f55962307eec2c2 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
@@ -2076,6 +2076,13 @@ void LayoutObject::mapLocalToAncestor(const LayoutBoxModelObject* ancestor,
   }
 
   LayoutSize containerOffset = offsetFromContainer(container);
+
+  // TODO(smcgruer): This is inefficient. Instead we should avoid including
+  // offsetForInFlowPosition in offsetFromContainer when ignoring sticky.
+  if (mode & IgnoreStickyOffset && isBoxModelObject() && isStickyPositioned()) {

[flackr, 2017/02/24 15:07:37]

I don't think the object can be sticky position and not a box model object?

[smcgruer, 2017/02/24 15:27:57]

Done.

+    containerOffset -= toLayoutBoxModelObject(this)->offsetForInFlowPosition();
+  }
+
   if (isLayoutFlowThread()) {
     // So far the point has been in flow thread coordinates (i.e. as if
     // everything in the fragmentation context lived in one tall single column).