function toggle_menu(){
  var menu = document.querySelector('#mobile-nav');
  menu.classList.toggle('open');
}

$( document ).ready(function() {
    var mainnav = $("#main-nav");
    var platzhalter = $("#platzhalter");
    var test = 2;

    $(window).on("scroll", function(e) {
        var pos = $(document).scrollTop();
        var width = $(window).width();
        var airAbove = 250;
        if (width < 1000) {
            if (width > 750) {
                airAbove = width/4;
            } else {
                airAbove = 187;
            }
        }

        if (pos > airAbove) {
            mainnav.addClass("fixed");
            platzhalter.addClass("fixed");
        } else {
            mainnav.removeClass("fixed");
            platzhalter.removeClass("fixed");
        }
    });
});