js实现fixed布局的按钮条儿，仿gmail
<html>
<head>
<title>fixed bar demo</title>
<style type="text/css" media="screen">
body{margin:0;height:2000px;}
#top{background-color:#333;height:80px;}
#nav{background-color:#999;height:30px;position:absolute;top:80px;width:100%;}
</style>
<script src="jquery.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript" charset="utf-8">
$(function(){
$(window).scroll(function(){
var scrollTop = $(window).scrollTop();
if(scrollTop < 80)
$("#nav").css('top', '80px');
else
$("#nav").css('top', scrollTop +'px');
});
});
</script>
</head>
<body>
<div id="top">
</div>
<div id="nav">
</div>
</body>
</html>
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