Unify the calculation of main thread offset of sticky element

cc/trees/property_tree.cc

 // Copyright 2014 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 <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 360 matching lines @@
     // The scroll position does not include snapping which shifts the scroll
     // offset to align to a pixel boundary, we need to manually include it here.
     // In this case, snapping is caused by a scroll.
     scroll_position -= transform_node->snap_amount;
   }
 
   gfx::RectF clip(
       scroll_position,
       gfx::SizeF(property_trees.scroll_tree.scroll_clip_layer_bounds(
           scroll_node->id)));
-  gfx::Vector2dF layer_offset(sticky_data->main_thread_offset);
 
   gfx::Vector2dF ancestor_sticky_box_offset;
   if (sticky_data->nearest_node_shifting_sticky_box !=
       TransformTree::kInvalidNodeId) {
     ancestor_sticky_box_offset =
         tree->StickyPositionData(sticky_data->nearest_node_shifting_sticky_box)
             ->total_sticky_box_sticky_offset;
   }
 
   gfx::Vector2dF ancestor_containing_block_offset;
@@ skipping 64 matching lines @@
     sticky_offset.set_y(sticky_offset.y() + top_delta);
   }
 
   sticky_data->total_sticky_box_sticky_offset =
       ancestor_sticky_box_offset + sticky_offset -
       sticky_box_rect.OffsetFromOrigin();
   sticky_data->total_containing_block_sticky_offset =
       ancestor_sticky_box_offset + ancestor_containing_block_offset +
       sticky_offset - sticky_box_rect.OffsetFromOrigin();
 
-  return sticky_offset - layer_offset - node->source_to_parent -
+  return sticky_offset - node->sticky_main_thread_offset -
          sticky_box_rect.OffsetFromOrigin();

[trchen, 2017/06/05 22:43:11]

This is quite confusing. I think the logic is to compute the local sticky offset
using only layout information (prior to blink adjustment), then subtract blink
adjustment from target adjustment, i.e. 

(sticky_offset - sticky_box_rect.OffsetFromOrigin()) -
node->sticky_main_thread_offset

which is the same value, but with correct associating order.

However it raised the question why sticky_offset included
sticky_box_rect.OffsetFromOrigin() in the first place. Can we just do

gfx::Vector2dF sticky_offset;

on line #408? Then we can replace "sticky_offset -
sticky_box_rect.OffsetFromOrigin()" with just "sticky_offset" on line #458 #461
#465.

[yigu, 2017/06/06 19:33:18]

Done.

 }
 
 void TransformTree::UpdateLocalTransform(TransformNode* node) {
   gfx::Transform transform = node->post_local;
   if (NeedsSourceToParentUpdate(node)) {
     gfx::Transform to_parent;
     ComputeTranslation(node->source_node_id, node->parent_id, &to_parent);
     gfx::Vector2dF unsnapping;
     TransformNode* current;
     TransformNode* parent_node;
@@ skipping 1712 matching lines @@
   const EffectNode* effect_node = effect_tree.Node(effect_id);
 
   if (effect_node->surface_contents_scale.x() != 0.0 &&
       effect_node->surface_contents_scale.y() != 0.0)
     screen_space_transform.Scale(1.0 / effect_node->surface_contents_scale.x(),
                                  1.0 / effect_node->surface_contents_scale.y());
   return screen_space_transform;
 }
 
 }  // namespace cc