/**
 * display/hide grey overlay div
 */
function toggleGlobalOverlay()
{
  var go = document.getElementById('grey_overlay');
  var fr = document.getElementById('body');
  var go_v = go.style.visibility;

  // have to set height dynamically because css-100% would refer to visible browser-window only, not whole html-height
  go.style.height = fr.offsetHeight + "px";   // offsetHeight has only a getter

  if( !go_v || (go_v == 'hidden') )
    document.getElementById('grey_overlay').style.visibility = 'visible';
  else
    document.getElementById('grey_overlay').style.visibility = 'hidden';
}