STAY ON TOP

HTML

<div class="homepage-intro"></div>
<div class="wrapper">
    <div class="navHeader">Menu</div>
    <div class="content">some content here
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   ...

CSS

html, body {
    height: 100%;
    margin: 0;
    padding: 0;
}
.homepage-intro {
    height: 100%;
    background:yellow;
}
.wrapper {
    background:#FFF;
}
.content {
}
.navHeader {
    background:#AAA;
    width: 100%;
    height: 50px;
}
.navFix {
    position:fixed;
    top:0;
    left: 0
}

JavaScript

$(document).ready(function () {
    $(window).scroll(function () {
        var windowpos = $(window).scrollTop(),
            pos = $(window).height();

        if (windowpos >= pos) {
            $(".navHeader").addClass("navFix");
        } else {
            $(".navHeader").removeClass("navFix");
        }
    });

}); // end ready