You can do it with CSS only by creating your menu twice. It's not ideal but it gives you the opportunity have a different design for the menu once it's on top and you'll have nothing else than CSS, no jquery. Here is an example with DIV (you can of course change it to NAV if you prefer):
<div id="hiddenmenu">
THIS IS MY HIDDEN MENU
</div>
<div id="header">
Here is my header with a lot of text and my main menu
</div>
<div id="body">
MY BODY
</div>
And then have the following CSS:
#hiddenmenu {
position: fixed;
top: 0;
z-index:1;
}
#header {
top: 0;
position:absolute;
z-index:2;
}
#body {
padding-top: 80px;
position:absolute;
z-index: auto;
}
Here is a fiddle for you to see: https://jsfiddle.net/brghtk4z/1/