Makes sure that the focused element is within view.
function updateScroll () {
    if (!elements.li[0]) return;
    var height = elements.li[0].offsetHeight,
        top = height * ctrl.index,
        bot = top + height,
        hgt = elements.scroller.clientHeight,
        scrollTop = elements.scroller.scrollTop;
    if (top < scrollTop) {
      scrollTo(top);
    } else if (bot > scrollTop + hgt) {
      scrollTo(bot - hgt);
    }
  }