Consider transformed container's scroll offset when calculating fixed descendant's offset

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 350a96c44547cbcc058aa4e77a3511d97f2b72cb..9851b2938dfee723e9d5f6fc6abafff1e86d962d 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1382,6 +1382,11 @@ static inline const RenderLayer* accumulateOffsetTowardsAncestor(const RenderLay
             location += (fixedContainerCoords - ancestorCoords);
         } else {
             location += layer->location();
+            // The spec (http://dev.w3.org/csswg/css-transforms/#transform-rendering) doesn't say if a
+            // fixed-position element under a scrollable transformed element should scroll. However,
+            // other parts of blink scroll the fixed-position element, and the following keeps the consistency.
+            if (RenderLayerScrollableArea* scrollableArea = ancestorLayer->scrollableArea())

[Julien - ping for review, 2015/01/09 13:25:02]

Won't this add the RenderView's scrollOffset to the location as the check is
only if fixedPositionContainerLayer == ancestorLayer (not if you have
encountered a containing block for fixed positioned elements that is not the
RenderView).

[Xianzhu, 2015/01/09 17:54:16]

The RenderView case has been handled above (line 1340 to 1351). Added an assert
and comments here. The distance to that context too big to be easily discovered
by readers.

+                location -= LayoutSize(scrollableArea->scrollOffset());
         }
         return ancestorLayer;
     }