This is implementation design problem. The following scenario shows bug related with setFullScreenMode:
It seems it works only the first time a GameCanvas is created, try:
java -jar microemulator.jar http://jbit.sourceforge.net/files/JBit2.jad
select: Demos, IO, resize, Video.
everything is OK; then
press '1' then select: End, resize, Video
now you can only see the top Xs.