出来ios顶部导航掉下来问题

<script type="text/javascript">
setposition();
function setposition(){
    var ua = navigator.userAgent.toLowerCase();    
    if (/iphone|ipad|ipod/.test(ua)) {
        jQuery(".lo_bj").css("position","absolute");
    }else if (/android/.test(ua)) {
           jQuery(".lo_bj").css("position","fixed");
    }
}
</script>