$(window).scroll(function () {
  if($(window).scrollTop() > 98){
   $('#top-menu').css({"margin-top":"-100px", "position":"fixed"});
   $('#top-menu').hover(function () { $(this).css({"margin-top":"0"}); }, function () { $(this).css({"margin-top":"-100px"}); } );
  } else {
   $('#top-menu').css({"margin-top":"0", "position":"absolute"});
   $('#top-menu').hover(function () { $(this).css("margin-top","0"); }, function () { $(this).css("margin-top","0px"); } );
  }
 });