Undo scroll snaps in to_parent when NeedsSourceToParentUpdate.

cc/trees/property_tree.cc

Index: cc/trees/property_tree.cc
diff --git a/cc/trees/property_tree.cc b/cc/trees/property_tree.cc
index cae3c1f4d2a26e16ace84114a10eb162444137e8..c45ec7f843f97bb479d2bc1efa0b377949bdc17b 100644
--- a/cc/trees/property_tree.cc
+++ b/cc/trees/property_tree.cc
@@ -328,6 +328,15 @@ void TransformTree::UpdateLocalTransform(TransformNode* node) {
   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();
   }