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..e3ea827488c73a368b7a10b9d4f0a0f1ca65b148 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
@@ -2076,6 +2076,14 @@ 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 (isBoxModelObject() && isStickyPositioned() &&
+      (mode & IgnoreStickyOffset)) {

[flackr, 2017/02/24 01:33:33]

nit: Maybe do the mode check first since most of the time it will be false.

[smcgruer, 2017/02/24 14:17:43]

Done. Also removed the brackets to let it fit on one line; lmk if that's bad
style.

+    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).