$(document).ready(function(){
  if( $('#elements_list').length != 0 ){
    $(window).scroll(function () {
      var margin = $(document).scrollTop()-150;
      if(margin < 0){ margin = 0; }

      actual_margin = parseInt($('#elements_list').css('margin-top').replace('px',''));
      actual_height = parseInt($('#elements_list').css('height').replace('px',''));
    
      if( (margin < actual_margin) || (margin > (actual_margin+actual_height)) ){
        margin = margin + 'px';
        $("#elements_list").stop();
        $('#elements_list').animate({marginTop:margin},500);
      }
    });
  }
});