.afix{

position:fixed;

top:45px;

z-index:100;

}

.fix{

position:fixed;

top:0px;

z-index:100;

}

.parent {

background: white;

width: 100%;

border-bottom: 1px solid #FFF;

}

window.οnscrοll= function(){

//变量t是滚动条滚动时，距离顶部的距离

//var current =document.documentElement.scrollTop;

var currnet = document.documentElement.scrollTop || window.pageYOffset || document.body.scrollTop;

var obj = $('.parent')[0];

var hig =$('#source_container').height();

var isAndroid = u.indexOf('Android') > -1|| u.indexOf('Linux') > -1;

var css ='fix';

if(isAndroid){

css = 'afix';

}

if(hig-current<50){

var obj = $('.parent')[0];

$(obj).addClass(css);// ('position','fixed');

}else{

if($(obj).hasClass(css))

{

$(obj).removeClass(css);

}

}

}

 

 

<div class="div_body topcss">

<div class="xuanfu">

<div id="source_container">

</div>

<div class="parent">

<div class="sui"><i class="fa fa-user-circle-o" aria-hidden="true"></i><input class="weui-btn_primary weui-input " type="button" value="我也来说两句" id="comment"></div>

</div>

</div>

<div class="container">

@*<div class="comentcss"><a href="#" id="more">查看更多</a></div>//这个功能去掉*@

</div>

</div>