$.ajaxSetup({ 
  'beforeSend': function(xhr) {
    xhr.setRequestHeader("Accept","text/javascript")} 
});

function is_fixed_supported() {
  var container = document.body;
  
  if (document.createElement && container && container.appendChild && container.removeChild) {
    var el = document.createElement('div');
    
    if (!el.getBoundingClientRect) return null;
        
    el.innerHTML = 'x';
    el.style.cssText = 'position:fixed;top:100px;';
    container.appendChild(el);

    var originalHeight = container.style.height,
        originalScrollTop = container.scrollTop;

    container.style.height = '3000px';
    container.scrollTop = 500;

    var elementTop = el.getBoundingClientRect().top;
    container.style.height = originalHeight;
    
    var isSupported = (elementTop === 100);
    container.removeChild(el);
    container.scrollTop = originalScrollTop;

    return isSupported;
  }
  return null;
}