Merge pull request matrix-org#4171 from matrix-org/jryans/relative-scrolling-release

Adjust scroll offset with relative scrolling (for release)

Author: jryans

src/components/structures/ScrollPanel.js

@@ -523,7 +523,7 @@ export default createReactClass({
  scrollRelative: function(mult) {
  const scrollNode = this._getScrollNode();
  const delta = mult * scrollNode.clientHeight * 0.5;
- scrollNode.scrollTop = scrollNode.scrollTop + delta;
+ scrollNode.scrollBy(0, delta);
  this._saveScrollState();
  },
 
@@ -705,17 +705,15 @@ export default createReactClass({
  // 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 = preexistingScrollTop + topDiff;
- debuglog("updateHeight to", {newHeight, topDiff, preexistingScrollTop});
+ // important to scroll by a relative amount as
+ // reading scrollTop and then setting it might
+ // yield out of date values and cause a jump
+ // when setting it
+ sn.scrollBy(0, topDiff);
+ debuglog("updateHeight to", {newHeight, topDiff});
  }
  }
  },
@@ -767,6 +765,7 @@ export default createReactClass({
  },
 
  _topFromBottom(node) {
+ // current capped height - distance from top = distance from bottom of container to top of tracked element
  return this._itemlist.current.clientHeight - node.offsetTop;
  },