window.onscroll = function(){ //缁戝畾scroll浜嬩欢
    var t = document.documentElement.scrollTop || document.body.scrollTop;  //鑾峰彇婊氬姩璺濈
    var h = $(window).height();
    var top_div = document.getElementById( "zptab_box" ); //鏌ヨ骞跺畾涔塪iv鍏冪礌/
    if( t >= 100 ) { //鍒ゆ柇
      $(top_div).addClass("protab"); 
    } else { 
      $(top_div).removeClass("protab"); 
    }
  };