(function(){
    document.observe ( 'dom:loaded' , function () {
        var navbar = $$(".navbar");
        if(navbar.length<1)
        return;
        var offsetTopx = navbar[0].offsetTop;
        window.onscroll = function(){
            var scrollY = window.pageYOffset || document.documentElement.scrollTop;
            if(scrollY>= offsetTopx)
            { navbar.invoke ( 'addClassName' , 'fixed_navbar' );}
            else
            { navbar.invoke ( 'removeClassName' , 'fixed_navbar' );}
        }
    });
})();

.fixed_navbar {
    position: fixed;
    top: 0;
}