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 310 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;
     ComputeTransform(node->source_node_id, node->parent_id, &to_parent);
+    gfx::Vector2dF unsnapping;
+    TransformNode* current;
+    for (current = Node(node->source_node_id); current->id > node->parent_id;
+         current = parent(current))
+      unsnapping.Subtract(current->scroll_snap);
+    for (TransformNode* parent_node = Node(node->parent_id);
+         parent_node->id >= current->id; parent_node = parent(parent_node))
+      unsnapping.Subtract(parent_node->scroll_snap);

[ajuma, 2016/07/26 19:34:41]

This is the right general approach, but I think there are some more details to
handle in order for this to work for the scroll parent case. 
1) I'm not sure it's true that the lowest ancestor of the source node whose id
is <= the parent node will also be an ancestor of the parent node. We used to
have code to find the lowest common ancestor of a pair of nodes but it was
removed in https://codereview.chromium.org/1070133002. I think we'll need
something like that.
2) When traversing the path from the lowest common ancestor to the parent, I
think you need to add rather than subtract?

Adding DCHECKs for the assumptions you're using might help to find tests with
examples where these assumptions don't hold.

Please also add LayerTreeHostCommonTests (for both the fixed-pos and scroll
parent cases) where the computed screen space transform changes with this patch.

+    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 1637 matching lines @@
   if (transform_id > destination_transform_id) {
     return transform_tree.CombineTransformsBetween(
         transform_id, destination_transform_id, transform);
   }
 
   return transform_tree.CombineInversesBetween(
       transform_id, destination_transform_id, transform);
 }
 
 }  // namespace cc