问当向元素分配id时，jQuery插件停止工作

EN

<section id="screen1">

    <p>Scroll down</p>

    <nav id="main-nav">
        <ul>
            <li><a href="#">Home</a></li>
            <li><a href="#">About</a></li>
            <li><a href="#">Services</a></li>
            <li><a href="#">Team</a></li>
            <li><a href="#">Contact</a></li>
        </ul>
    </nav>

</section>

<section id="screen2"></section>
<section id="screen3"></section>

/* Navigation Settings */

#main-nav {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 70px;
    background: #fff;
}

$(document).ready(function(){
    $(window).bind('scroll', function() {
        var navHeight = $( window ).height() - 70;
        if ($(window).scrollTop() > navHeight) {
            $('#main-nav').addClass('fixed');
        }
        else {
            $('#main-nav').removeClass('fixed');
        }
    });
});