Setting a menu bar after a maximzed QMdiSubWindow is added causes the title icons to dissappear

If a QMenuBar is added to a QMainWindow after this window has added a QMdiArea containing a maximized subwindow as its central widget, then the title bar of the sub window should be merged in to the menu bar. This is not the case at the moment because the title bar is not merged in unless the window is maximized after the menubar has been added.