function scroll(scrollAxis, shift) {
      var property = scrollAxis == 'vertical' ? 'scrollTop' : 'scrollLeft';
      el[property] += shift;
    }
Scrolls the selection container viewport by the required amount.

@param {string} scrollAxis The scrolling axis, either 'vertical' or
    'horizontal'.
@param {number} shift The scrolling amount, in pixels. If positive the
    scrolling direction should be downward / rightward. If negative,
    upward / leftward.