Let\'s get the fiddles out of the way first (built on the Foundation responsive framework):
Fiddle 1: one nav element, but becomes unfixed when side nav slides out>
I would suggest moving the element, outside of the div#page. That way any styling you put on the page won't affect it (including the translations), and you are free to style your nav list as needed.
http://jsfiddle.net/mKAtM/3/
This was just a quick test, i've moved the nav out of the div as suggested and added styles along the lines of:
.top-bar {
z-index:1000;
}
.top-bar.expanded{
-webkit-transform: translate3d(250px, 0, 0);
-moz-transform: translate3d(250px, 0, 0);
-o-transform: translate3d(250px, 0, 0);
transform: translate3d(250px, 0, 0);
overflow:visible;
}
(Think the overflow:visible is a different issue)
Obviously you need to add your transitions etc. and the translates should be in a class, but hope this is a step in the right direction
EDIT
updated fiddle to fix animation: http://jsfiddle.net/mKAtM/8/
css needs some tidying up, much easier to with your less/scss if you are using