QMdiArea should expose tabbar widget in order to provide more control over the tabbar in tabbed view mode
the implementation may be as simple as

QTabBar *QMdiArea::tabBar()
{ return d_func()->tabBar; }

furthermore it would be nice to have a kind of 'resetSystemMenu()' member function in the QMdiSubWindow public interface
an implementation can be like this:

void QMdiSubWindow::resetSystemMenu()
{
  Q_D(QMdiSubWindow);
  if (d->systemMenu) delete d->systemMenu;
 d->createSystemMenu();
}