sinppet for docked menu.

docked_menu_sinppet.js

    var menu = document.getElementById('menu');
    var init = menu.offsetTop;
    var docked;

    window.onscroll = function () {
      if (!docked && (menu.offsetTop - scrollTop() < 0)) {
        menu.style.top = 0;
        menu.style.position = 'fixed';
        menu.className = 'docked';
        docked = true;
      } else if (docked && scrollTop() <= init) {
        menu.style.position = 'absolute';
        menu.style.top = init + 'px';
        menu.className = menu.className.replace('docked', '');
        docked = false;
      }
    };

    function scrollTop() {
        return document.body.scrollTop || document.documentElement.scrollTop;
    }