Notification messages are displayed at the bottom of the page instead of bottom of the screen

Steps to reproduce:

• create an anchor element and add a lot of dummy text to have the scroll enabled:

  <a id="example" href="#Example">here</a> Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed sed eros rhoncus dui volutpat rutrum sit amet non erat. Sed eleifend, odio in lobortis lobortis, quam est ornare nisi, vel ...
  

• add a notification for the click event using a jsx extension:

  document.observe("xwiki:dom:loaded", function() {
    if($('example')) {
      $('example').observe('click', function(event) {
        new XWiki.widgets.Notification('test', 'done');
      });
    }
  });