None of the sub-menus open up when pressing the respective mnemonics. Same applies to popup menu as well. Mnemonics work only for the menu items. This is a regression introduced in the current AWT PIT build - b71. This works fine on b69.

This is reproducible on all platforms. There is a mnemonic related fix in the current PIT build:
6363026: Mnemonics don't consume proper events
Not sure whether this regression is caused by the above fix.

This is a basic feature in Swing and hence it is a blocker.

To reproduce the bug, run the attached test. Press 'ALT + G'. The menu opens up. Press 'F'. If the sub-menu does not open up, the bug is reproduced. You can reproduce it for popup menu by right clicking inside the button. A popup menu opens up. Try navigating through the popup menu using the mnemonics shown. If none of them are accessible, bug is reproduced.
This can also be reproduced using SwingSet2. Open any menu using the mnemonic key and press the mnemonic key of any of the sub-menus. If the sub-menu does not open up, the bug is reproduced.