顶部悬浮

window.onscroll = function(){

    var div2 = document.getElementById("div2");

    var _top = document.body.scrollTop || document.documentElement.scrollTop;

    if(_top > 400) {

        div2.style.position = "fixed";

        div2.style.top = "0px";

    } else {

        div2.style.position = "absolute";

        div2.style.top = "400px";

    }

}