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

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

JavaScript

$(document).ready(function () {

    var $navHeader = $(".navHeader");
    var $pos = $navHeader.position();

    $(window).scroll(function () {
        var windowpos = $(window).scrollTop();

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

}); // end ready