当用户滚动到特定位置时，
或者当他开始滚动时，您可以通过 jQuery 添加一个类.

When the user scolls to an specific position,
or in your case when he begins to scroll, you can add an class via jQuery.

在此处查看更多信息:

滚动时将菜单栏固定在顶部

(代码取自链接中的帖子)

(Code is taken from the post in the link)

$(window).bind('scroll', function() {
     if ($(window).scrollTop() > 50) {
         $('.menu').addClass('fixed');
     }
     else {
         $('.menu').removeClass('fixed');
     }
});

当你用你的代码创建一个 fiddle 是***的，这样其他人就可以轻松修改你的代码和看看那里发生了什么..

it is the best when you create an fiddle with your code, so other people can easy modify your code and see whats going on there..