show-header-menu.html

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<script>
var lastScrollTop = 0;
$(window).scroll(function(event){
   var st = $(this).scrollTop();

   if (st > lastScrollTop){
       // downscroll code
      $('#site-navigation').removeClass('fixed-menu');
   } else {
      // upscroll code
      
      $('#site-navigation').addClass('fixed-menu');
   }
   lastScrollTop = st;
});</script>