Unify the calculation of main thread offset of sticky element

third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef StickyPositionScrollingConstraints_h
 #define StickyPositionScrollingConstraints_h
 
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/FloatSize.h"
+#include "platform/wtf/HashMap.h"
 
 namespace blink {
 
 class LayoutBoxModelObject;
+class PaintLayer;
+class StickyPositionScrollingConstraints;
 
+typedef WTF::HashMap<PaintLayer*, StickyPositionScrollingConstraints>
+    StickyConstraintsMap;
+
+// TODO(smcgruer): Add detailed comment explaining how
+// StickyPositionScrollingConstraints works.
 class StickyPositionScrollingConstraints final {
  public:
   enum AnchorEdgeFlags {
     kAnchorEdgeLeft = 1 << 0,
     kAnchorEdgeRight = 1 << 1,
     kAnchorEdgeTop = 1 << 2,
     kAnchorEdgeBottom = 1 << 3
   };
   typedef unsigned AnchorEdges;
 
@@ skipping 81 matching lines @@
     return nearest_sticky_box_shifting_containing_block_;
   }
 
   const FloatSize& GetTotalStickyBoxStickyOffset() const {
     return total_sticky_box_sticky_offset_;
   }
   const FloatSize& GetTotalContainingBlockStickyOffset() const {
     return total_containing_block_sticky_offset_;
   }
 
+  // Returns the relative position of the sticky box and its original position
+  // before scroll. This method is only safe to call if ComputeStickyOffset has
+  // been invoked.
+  FloatSize GetOffsetForStickyPosition(const StickyConstraintsMap&) const;
+
   const LayoutBoxModelObject* NearestStickyAncestor() const {
     // If we have one or more sticky ancestor elements between ourselves and our
     // containing block, |m_nearestStickyBoxShiftingStickyBox| points to the
     // closest. Otherwise, |m_nearestStickyBoxShiftingContainingBlock| points
     // to the the first sticky ancestor between our containing block (inclusive)
     // and our scroll ancestor (exclusive). Therefore our nearest sticky
     // ancestor is the former if it exists, or the latter otherwise.
     //
     // If both are null, then we have no sticky ancestors before our scroll
     // ancestor, so the correct action is to return null.
@@ skipping 51 matching lines @@
   // constraining block constriant rect for different sticky descendants is
   // quite complex. See the StickyPositionComplexTableNesting test in
   // LayoutBoxModelObjectTest.cpp.
   FloatSize total_sticky_box_sticky_offset_;
   FloatSize total_containing_block_sticky_offset_;
 };
 
 }  // namespace blink
 
 #endif  // StickyPositionScrollingConstraints_h