[ruslan_mart]

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

(function() {
	var menu = document.querySelector('.menu'),
		menuIsFixed = false,
		menuMaxTop = 400;
	
	window.addEventListener('scroll', function() {
		if(this.pageYOffset >= menuMaxTop != menuIsFixed) {
			menuIsFixed = !menuIsFixed;
			menu.classList[menuIsFixed ? 'add' : 'remove']('fixed');
		}
	});
})();