Unify the calculation of main thread offset of sticky element

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

 // 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.
 
 #include "core/page/scrolling/StickyPositionScrollingConstraints.h"
+#include "core/paint/PaintLayer.h"
 
 namespace blink {
 
 FloatSize StickyPositionScrollingConstraints::ComputeStickyOffset(
     const FloatRect& viewport_rect,
     const StickyPositionScrollingConstraints* ancestor_sticky_box_constraints,
     const StickyPositionScrollingConstraints*
         ancestor_containing_block_constraints) {
   // Adjust the constraint rect locations based on our ancestor sticky elements
   // These adjustments are necessary to avoid double offsetting in the case of
@@ skipping 65 matching lines @@
   FloatSize sticky_offset = box_rect.Location() - sticky_box_rect.Location();
 
   total_sticky_box_sticky_offset_ = ancestor_sticky_box_offset + sticky_offset;
   total_containing_block_sticky_offset_ = ancestor_sticky_box_offset +
                                           ancestor_containing_block_offset +
                                           sticky_offset;
 
   return sticky_offset;
 }
 
+FloatSize StickyPositionScrollingConstraints::GetOffsetForStickyPosition(
+    const StickyConstraintsMap& constraints_map) const {
+  FloatSize nearest_sticky_box_shifting_sticky_box_constraints_offset;
+  if (nearest_sticky_box_shifting_sticky_box_) {
+    nearest_sticky_box_shifting_sticky_box_constraints_offset =
+        constraints_map.at(nearest_sticky_box_shifting_sticky_box_->Layer())
+            .GetTotalStickyBoxStickyOffset();
+  }
+  return total_sticky_box_sticky_offset_ -
+         nearest_sticky_box_shifting_sticky_box_constraints_offset;
+}
+
 }  // namespace blink