message: Add comment to _scrollToToken
diff: @@ -555,6 +555,9 @@ module.exports = React.createClass({
  var messages = this.refs.itemlist.children;
  for (var i = messages.length-1; i >= 0; --i) {
  var m = messages[i];
  + // 'data-contained-scroll-tokens' has been set, indicating that a child
  + // element contains elements that each have a token. Check this array of
  + // tokens for `scrollToken`.
  if (m.dataset.containedScrollTokens &&
  m.dataset.containedScrollTokens.indexOf(scrollToken) !== -1) {
  node = m;
  