window.onscroll = function(){ //绑定scroll事件
    var t = document.documentElement.scrollTop || document.body.scrollTop;  //获取滚动距离
    var h = $(window).height();
    var top_div = document.getElementById( "w_grid-1558956156232" ); //查询并定义div元素/
    if( t >= 700 ) { //判断
      $(top_div).addClass("ztest"); 
    } else { 
      $(top_div).removeClass("ztest"); 
    }
  };