Breakthrough: I think this is caused by the chatlogger window, which gets given the active focus at bootup instead of the main JJ2 window, which means that the chatlogger receives key inputs instead of JJ2. This suggests two solutions that should allow people to type letters in fullscreen:

• Running JJ2 with the -nochatlogger command line argument, and/or with [Logging]Chatlog = False in plus.ini
• This test build puts JJ2 in front of the chatlogger