I can reproduce the problem on my Windows laptop when I use a clean installation and
java-options=-Dsun.java2d.uiScale=2
in the cfg. BTW: when I click ESC and F12 again the dialog shows the lower part with the buttons.
To be honest, I don't know why we use the setMinimumSize() at all. For me, JOSM works fine when I remove the statement, both with and without the sun.java2d.uiScale=2. Maybe it is needed with other OS or older JRE?