function scroll(scrollAxis, shift) {
      if (scrollAxis == 'vertical') {
        window.scrollBy(0, shift);
      } else {
        window.scrollBy(shift, 0);
      }
    }
Scrolls the browser window 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.