The border zone is now displayed (Hbor, Vbor) :
I've not done yet the version with the filled rectangle. For the 'outline' version, i wanted also to make lines with a soft shadow or glow instead of black and white lines, but it doesn't seem trivial with Swing. You can find a library like swingx which can do this, but it seems overkill to me to use a library just for one feature.