页面滚动时出现固定导航

页面滚动时出现固定导航的效果，参考页面有http://v.thea.cn/course/info_358和v.thea.cn.

参考代码如下：

<div class="tab-title" style="position: static; top: 0px;"><span class="normal selected">课程介绍</span><span class="normal">课程大纲</span><span class="normal">课程评论</span>
<span style="display: none;" class="spa-try-listen "><a target="_blank" href="/play/358_3018">免费试听</a></span>
<span "="" style="display: none;" class="normal"><a href="/Cart/addCart/courseid/358/coursetype/0/wx/thea">购买课程</a></span><!-- /pay/course_358 --> </div>

js如下：

$(window).scroll(function(){ 
		var scroll_offset = $(".pcd_main .tab").offset().top;
		var t = document.documentElement.scrollTop || document.body.scrollTop;
		//var $tabtitle=$(".pcd_main .tab-title");
		//var $liLast=$tabtitle.children("span").last();
		if(t>scroll_offset){
			$tabtitle.css({"position":"fixed","top":"0"});
			//$liLast.removeClass("last");
			//$tabtitle.find(".spa-try-listen").show().next().show();
		}else{
			$tabtitle.css("position","static");
			//$liLast.removeClass("last");
			//$tabtitle.find(".spa-try-listen").hide().next().hide();
		}
	});