var timer

window.onload = scrolltop;
window.onunload = stoptimer;

function scrolltop() {
  var menu = document.getElementById('scrollmenu');
  var scrollTop = document.body.scrollTop || document.documentElement.scrollTop
  menu.style.top = scrollTop;
  timer = setTimeout("scrolltop()",10);
}
function stoptimer() {
  clearTimeout(timer)
}