message: fix jumping when unfilling the top while scrolling down
diff: @@ -642,11 +642,17 @@ module.exports = React.createClass({
  // the currently filled piece of the timeline
  if (trackedNode) {
  const oldTop = trackedNode.offsetTop;
  + // changing the height might change the scrollTop
  + // if the new height is smaller than the scrollTop.
  + // We calculate the diff that needs to be applied
  + // ourselves, so be sure to measure the
  + // scrollTop before changing the height.
  + const preexistingScrollTop = sn.scrollTop;
  itemlist.style.height = `${newHeight}px`;
  const newTop = trackedNode.offsetTop;
  const topDiff = newTop - oldTop;
  - sn.scrollTop = sn.scrollTop + topDiff;
  - debuglog("updateHeight to", newHeight, topDiff);
  + sn.scrollTop = preexistingScrollTop + topDiff;
  + debuglog("updateHeight to", {newHeight, topDiff, preexistingScrollTop});
  }
  }
  },
  