Disabling a top-level menu has no effect under MacOS/X

Disabling a top-level menu (i.e. one that appears in the QMenuBar) doesn't work under MacOS/X (Cocoa or Carbon).

To reproduce:

1) Edit $QTDIR/examples/widgets/imageviewer/imageviewer.cpp
2) At line 227 of that file, add this line:

fileMenu->menuAction()->setEnabled(false); // disable the File menu!

3) Compile and run the program

Note that under Linux and Windows, the File menu is disabled, this is the behavior I expected. Under MacOS/X, however, the File menu remains enabled; I think that is a bug.