Come si evince dalla documentazione http://docs.oracle.com/javase/tutori...enu.html#popup
codice:
The exact gesture that should bring up a popup menu varies 
by look and feel. In Microsoft Windows, the user by convention brings up 
a popup menu by releasing the right mouse button while the cursor is over 
a component that is popup-enabled. In the Java look and feel, the customary 
trigger is either pressing the right mouse button (for a popup that goes away 
when the button is released) or clicking it (for a popup that stays up).
Quindi devi gestire meglio tutti gli eventi del mouse.