$(document).ready(function(){
window.onscroll = function(){
var forum_main = document.getElementById("forum_main");
var thetop =document.documentElement.scrollTop + document.body.scrollTop;
var forum_box_top = forum_main.offsetTop;
//window.console && console.log(forum_box_top);
//window.console && console.log(thetop);
if(thetop >= forum_box_top){
$('#forum_float_box').addClass('twfir');
}else{
$('#forum_float_box').removeClass('twfir');
}
}
})
![](http://images0.cnblogs.com/blog/572766/201409/171820059404324.png)