gistfile1.m

- (void)setDocument:(CPDocument)aDocument
{
  if (aDocument === _document)
    return;

  [super setDocument:aDocument];

  var unloadFunction = function() {
    if ([_document isDocumentEdited]) {
      return "Your document has unsaved changes.  If you leave, you will lose any unsaved changes to your document.";
    }
  };
  if (document.attachEvent)
    [[self window] platformWindow]._DOMWindow.attachEvent("onbeforeunload", unloadFunction);
  else 
    [[self window] platformWindow]._DOMWindow.onbeforeunload = unloadFunction;
}