// https://stackoverflow.com/questions/9236314/how-do-i-synchronize-the-scroll-position-of-two-divs/9236351

var isSyncingLeftScroll = false;
var isSyncingRightScroll = false;
var leftDiv = document.getElementById('left');
var rightDiv = document.getElementById('right');

leftDiv.onscroll = function() {
  if (!isSyncingLeftScroll) {
    isSyncingRightScroll = true;
    rightDiv.scrollTop = this.scrollTop;
  }
  isSyncingLeftScroll = false;
}

rightDiv.onscroll = function() {
  if (!isSyncingRightScroll) {
    isSyncingLeftScroll = true;
    leftDiv.scrollTop = this.scrollTop;
  }
  isSyncingRightScroll = false;
}