Undo scroll snaps in to_parent when NeedsSourceToParentUpdate.

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 319 matching lines @@
   bool all_are_invertible = dest_to_source.GetInverse(&source_to_dest);
   transform->PreconcatTransform(source_to_dest);
   return all_are_invertible;
 }
 
 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;
+    for (current = Node(node->source_node_id); current->id > node->parent_id;
+         current = parent(current)) {
+      unsnapping.Subtract(current->scroll_snap);
+    }
+    for (parent_node = Node(node->parent_id);
+         parent_node->id > node->source_node_id;
+         parent_node = parent(parent_node)) {
+      unsnapping.Add(parent_node->scroll_snap);
+    }
+    // If a node NeedsSourceToParentUpdate, the node is either a fixed position
+    // node or a scroll child.
+    // If the node has a fixed position, the parent of the node is an ancestor
+    // of source node, current->id should be equal to node->parent_id.
+    // Otherwise, the node's source node is always an ancestor of the node owned
+    // by the scroll parent, so parent_node->id should be equal to
+    // node->source_node_id.
+    DCHECK(current->id == node->parent_id ||
+           parent_node->id == node->source_node_id);
+    to_parent.Translate(unsnapping.x(), unsnapping.y());
     node->source_to_parent = to_parent.To2dTranslation();
   }
 
   gfx::Vector2dF fixed_position_adjustment;
   gfx::Vector2dF inner_viewport_bounds_delta =
       property_trees()->inner_viewport_container_bounds_delta();
   gfx::Vector2dF outer_viewport_bounds_delta =
       property_trees()->outer_viewport_container_bounds_delta();
   if (node->affected_by_inner_viewport_bounds_delta_x)
     fixed_position_adjustment.set_x(inner_viewport_bounds_delta.x());
@@ skipping 1654 matching lines @@
     DCHECK(effect_node->has_render_surface ||
            effect_node->id == EffectTree::kRootNodeId);
     target_transform_id = effect_node->transform_id;
   }
 
   return transform_tree.ComputeTransform(target_transform_id, transform_id,
                                          transform);
 }
 
 }  // namespace cc