I’m slightly out of ideas here where to even start looking for this issue. Other Java applications are looking fine. Any starting points would be appreciated…
Which OS and which Java version are you using? Here (Ubuntu 12.04, Java 1.7.0_25) everything looks fine.
Gentoo Linux on AMD64 with i3, same version of the JRE as you. This has been broken on this machine for a long time.
Weirdly, yesterday, during the FIRST run after upgrading from the .jar to the WebStart version everything looked ok again. Until, well, it did the same thing again.
Actually the WebStart version doesn’t run for me at all, and simply crashes at startup with an exception related to font rendering… So I guess there is indeed a serious problem. Does the .jar version work for you?
No, the jar version looks the same, switching between Java 1.6 and 1.7 doesn’t make a difference either.
Have you tried resetting your preferences? You can do this by renaming/deleting the preferences directory. See http://josm.openstreetmap.de/wiki/Help/ResetPreferences