message: restore scroll state when timeline resizes using ResizeObserver
  (only where supported, polyfill doesn't give good results)
diff: @@ -160,6 +160,13 @@ module.exports = React.createClass({
  componentDidMount: function() {
  this.checkScroll();
  +
  + if (typeof ResizeObserver !== "undefined") {
  + this._timelineSizeObserver = new ResizeObserver(() => {
  + this._restoreSavedScrollState();
  + });
  + this._timelineSizeObserver.observe(this.refs.itemlist);
  + }
  },
  componentDidUpdate: function() {
  @@ -181,6 +188,10 @@ module.exports = React.createClass({
  //
  // (We could use isMounted(), but facebook have deprecated that.)
  this.unmounted = true;
  + if (this._timelineSizeObserver) {
  + this._timelineSizeObserver.disconnect();
  + this._timelineSizeObserver = null;
  + }
  },
  onScroll: function(ev) {
  