Compute and include the offset of the sticky box to its enclosing composited layer.

third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h

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 #ifndef WebLayerStickyPositionConstraint_h
 #define WebLayerStickyPositionConstraint_h
 
+#include "public/platform/WebPoint.h"
+#include "public/platform/WebRect.h"
+
 namespace blink {
 
 // TODO(flackr): Combine with WebLayerPositionConstraint.
 struct WebLayerStickyPositionConstraint {
   // True if the layer is sticky.
   bool isSticky : 1;
 
   // For each edge, true if the layer should stick to that edge of its
   // ancestor scroller (or the viewport).
   bool isAnchoredLeft : 1;
   bool isAnchoredRight : 1;
   bool isAnchoredTop : 1;
   bool isAnchoredBottom : 1;
 
   // For each edge, when the anchored bit above is set this gives the distance
   // to keep the element from that edge of the ancestor scroller or viewport.
   float leftOffset;
   float rightOffset;
   float topOffset;
   float bottomOffset;
 
+  // This is the layout position of the sticky element before it has been
+  // shifted relative to the enclosing composited layer.
+  WebPoint parentRelativeStickyBoxOffset;
+
   // The layout rectangle of the sticky element before it has been shifted
   // to stick.
   WebRect scrollContainerRelativeStickyBoxRect;
 
   // The layout rectangle of the containing block edges which this sticky
   // element should not be shifted beyond.
   WebRect scrollContainerRelativeContainingBlockRect;
 
   WebLayerStickyPositionConstraint()
       : isSticky(false),
         isAnchoredLeft(false),
         isAnchoredRight(false),
         isAnchoredTop(false),
         isAnchoredBottom(false),
         leftOffset(0.f),
         rightOffset(0.f),
         topOffset(0.f),
         bottomOffset(0.f) {}
 };
 
 }  // namespace blink
 
 #endif  // WebLayerStickyPositionConstraint_h