Handle nested position:sticky elements correctly (compositor)

cc/layers/layer_sticky_position_constraint.cc

 // Copyright 2013 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 "cc/layers/layer_sticky_position_constraint.h"
 
+#include "cc/layers/layer.h"
+
 namespace cc {
 
 LayerStickyPositionConstraint::LayerStickyPositionConstraint()
     : is_sticky(false),
       is_anchored_left(false),
       is_anchored_right(false),
       is_anchored_top(false),
       is_anchored_bottom(false),
       left_offset(0.f),
       right_offset(0.f),
       top_offset(0.f),
-      bottom_offset(0.f) {}
+      bottom_offset(0.f),
+      nearest_layer_shifting_sticky_box(Layer::INVALID_ID),
+      nearest_layer_shifting_containing_block(Layer::INVALID_ID) {}
 
 LayerStickyPositionConstraint::LayerStickyPositionConstraint(
     const LayerStickyPositionConstraint& other)
     : is_sticky(other.is_sticky),
       is_anchored_left(other.is_anchored_left),
       is_anchored_right(other.is_anchored_right),
       is_anchored_top(other.is_anchored_top),
       is_anchored_bottom(other.is_anchored_bottom),
       left_offset(other.left_offset),
       right_offset(other.right_offset),
       top_offset(other.top_offset),
       bottom_offset(other.bottom_offset),
       parent_relative_sticky_box_offset(
           other.parent_relative_sticky_box_offset),
       scroll_container_relative_sticky_box_rect(
           other.scroll_container_relative_sticky_box_rect),
       scroll_container_relative_containing_block_rect(
-          other.scroll_container_relative_containing_block_rect) {}
+          other.scroll_container_relative_containing_block_rect),
+      nearest_layer_shifting_sticky_box(
+          other.nearest_layer_shifting_sticky_box),
+      nearest_layer_shifting_containing_block(
+          other.nearest_layer_shifting_containing_block) {}
 
 bool LayerStickyPositionConstraint::operator==(
     const LayerStickyPositionConstraint& other) const {
   if (!is_sticky && !other.is_sticky)
     return true;
   return is_sticky == other.is_sticky &&
          is_anchored_left == other.is_anchored_left &&
          is_anchored_right == other.is_anchored_right &&
          is_anchored_top == other.is_anchored_top &&
          is_anchored_bottom == other.is_anchored_bottom &&
          left_offset == other.left_offset &&
          right_offset == other.right_offset && top_offset == other.top_offset &&
          bottom_offset == other.bottom_offset &&
          parent_relative_sticky_box_offset ==
              other.parent_relative_sticky_box_offset &&
          scroll_container_relative_sticky_box_rect ==
              other.scroll_container_relative_sticky_box_rect &&
          scroll_container_relative_containing_block_rect ==
-             other.scroll_container_relative_containing_block_rect;
+             other.scroll_container_relative_containing_block_rect &&
+         nearest_layer_shifting_sticky_box ==
+             other.nearest_layer_shifting_sticky_box &&
+         nearest_layer_shifting_containing_block ==
+             other.nearest_layer_shifting_containing_block;
 }
 
 bool LayerStickyPositionConstraint::operator!=(
     const LayerStickyPositionConstraint& other) const {
   return !(*this == other);
 }
 
+int LayerStickyPositionConstraint::NearestStickyAncestor() {
+  return (nearest_layer_shifting_sticky_box != Layer::INVALID_ID)
+             ? nearest_layer_shifting_sticky_box
+             : nearest_layer_shifting_containing_block;
+}
+
 }  // namespace cc