code: function _handleScroll() {
          // Save current scroll
          // Supports IE 9 and up.
          const nx = hasScrollContainer ? viewEl.scrollLeft : window.pageXOffset;
          const ny = hasScrollContainer ? viewEl.scrollTop : window.pageYOffset;
          scroll.setScroll(nx, ny);
  
          // Only called if the last animation request has been
          // completed and there are parallax elements to update
          if (!ticking && elements.length > 0) {
              ticking = true;
              window.requestAnimationFrame(_updateAllElements);
          }
      }
docs: Window scroll handler sets scroll position
  and then calls '_updateAllElements()'.