Unify the calculation of main thread offset of sticky element

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 824 matching lines @@
               .OffsetForInFlowPositionedInline(ToLayoutBox(GetLayoutObject()));
       local_point += offset;
     }
   }
 
   if (GetLayoutObject().IsInFlowPositioned()) {
     LayoutSize new_offset = GetLayoutObject().OffsetForInFlowPosition();
     if (rare_data_ || !new_offset.IsZero())
       EnsureRareData().offset_for_in_flow_position = new_offset;
     local_point.Move(new_offset);
+
+    // Find the layout offset of the unshifted sticky box within its parent
+    // composited layer. This information is used by the compositor side to
+    // compute the additional offset required to keep the element stuck under
+    // compositor scrolling.
+    if (GetLayoutObject().Style()->GetPosition() == EPosition::kSticky &&
+        GetCompositedLayerMapping()) {
+      GetCompositedLayerMapping()
+          ->MainGraphicsLayer()
+          ->SetStickyMainThreadOffset(RoundedIntSize(new_offset));

[flackr, 2017/06/01 19:01:47]

I don't think this is the right place to set the sticky main thread offset on
the graphics layer since it may be created later. We should be doing this when
we set the graphics layer's position based, which is I believe in
CompositedLayerMapping::UpdateMainGraphicsLayerGeometry.

[yigu, 2017/06/01 19:28:40]

Done.

+    }
   } else if (rare_data_) {
     rare_data_->offset_for_in_flow_position = LayoutSize();
   }
 
   location_ = local_point;
 
 #if DCHECK_IS_ON()
   needs_position_update_ = false;
 #endif
 }
@@ skipping 2489 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif