message: correctly calculate last node in viewport
  these variables are now relative to bottom of timeline,
  before it was the top
diff: @@ -555,7 +555,7 @@ module.exports = React.createClass({
  node = messages[i];
  // break at the first message (coming from the bottom)
  // that has it's offsetTop above the bottom of the viewport.
  - if (node.offsetTop < viewportBottom) {
  + if (this._topFromBottom(node) > viewportBottom) {
  // Use this node as the scrollToken
  break;
  }
  