docs: function start() {
    if (isStarted()) {
      return;
    }
  
    // Debounce the scroll event by executing the onScroll callback using a timed
    // interval.
    window.addEventListener('scroll', onScroll_, {'passive': true});
    interval = window.setInterval(function() {
      if (scrolled) {
        for (var i = 0, delegate; delegate = delegates[i]; i++) {
          delegate.onScroll();
        }
        scrolled = false;
      }
    }, 250);
  }