I'm not completely sure about this, not having tried it.  But given that
the JIF is generally used with a JDesktopPane, you might try using it
there instead.

In our app, we've got a JIF subclass implementing VetoableChangeListener
so that we can check whether the document is dirty and allow a user to  
save, not save or cancel.  The very first thing we do in that listener's
vetoableChange() method is check whether the property name matches the
JIF's IS_CLOSED_PROPERTY, and whether it's changing from false to true
to indicate closing.  There's no property listed for a JIF (at least in
1.4) that I can clearly associate with deactivation, unless it's maybe
the IS_SELECTED_PROPERTY, so you'd have to experiment with it.  But if
it's the correct property, then you can throw a PropertyVetoException
from within this method when conditions shouldn't allow the JIF to
deactivate.

If that won't work, then I guess some tinkering with your
InternalFrameListener might be in order.  There's typically a
DesktopManager somewhere that can be told to reactivate your JIF if it's
not supposed to give up its status.

HTH.

= Steve =