Event.observe(document.body, 'click', function(e)
{
  var eventXY = Event.pointer(e); // Event X/Y coords
 
  var dialogElement = $('dialog'); // Element dialog container
  var dialogXY = dialogElement.cumulativeOffset(); // Dialog X/Y coords
  var dialogWH = dialogElement.getDimensions(); // Dialog W/H
 
  var dialogXRange = $R(dialogXY[0], (dialogXY[0] + dialogWH.width));
  var dialogYRange = $R(dialogXY[1], (dialogXY[1] + dialogWH.height));
 
  if (dialogXRange.include(eventXY.x) && dialogYRange.include(eventXY.y))
    console.log('withtin element');
});