Unify the calculation of main thread offset of sticky element

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

Index: third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
index f099cdce40417819ed11a11f133cdf54b503a75f..76d5b67ae70c2cbe3db811296d95ee63ffa8e84c 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "core/page/scrolling/StickyPositionScrollingConstraints.h"
+#include "core/paint/PaintLayer.h"
 
 namespace blink {
 
@@ -88,4 +89,18 @@ FloatSize StickyPositionScrollingConstraints::ComputeStickyOffset(
   return sticky_offset;
 }
 
+FloatSize StickyPositionScrollingConstraints::ComputeMainThreadStickyOffset(
+    const PaintLayer& ancestor_overflow_layer) const {

[flackr, 2017/06/02 15:33:58]

Instead of passing in the ancestor_overflow_layer it'd probably have a cleaner
API if we just pass the StickyConstraintsMap.

[yigu, 2017/06/02 17:59:56]

Done.

+  FloatSize nearest_sticky_box_shifting_sticky_box_constraints_offset;
+  if (nearest_sticky_box_shifting_sticky_box_) {
+    nearest_sticky_box_shifting_sticky_box_constraints_offset =
+        ancestor_overflow_layer.GetScrollableArea()
+            ->GetStickyConstraintsMap()
+            .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