docs: function onScroll(body, evt) {
      if (body.scrollTop > 0) {
          $.addClass(body, "top");
      } else {
          $.removeClass(body, "top");
      }
  
      if (body.scrollHeight - body.scrollTop > body.clientHeight) {
          $.addClass(body, "bottom");
      } else {
          $.removeClass(body, "bottom");
      }
  }