message: take (potentially clipped) content height into account for filling
diff: @@ -298,11 +298,14 @@ module.exports = React.createClass({
  // `---------' -
  //
  - if (sn.scrollTop < sn.clientHeight) {
  + const contentHeight = this._getMessagesHeight();
  + const contentTop = contentHeight - this._getListHeight();
  + const contentScrollTop = sn.scrollTop + contentTop;
  + if (contentScrollTop < sn.clientHeight) {
  // need to back-fill
  this._maybeFill(true);
  }
  - if (sn.scrollTop > sn.scrollHeight - sn.clientHeight * 2) {
  + if (contentScrollTop > contentHeight - sn.clientHeight * 2) {
  // need to forward-fill
  this._maybeFill(false);
  }
  