The Java Look and Feel that Processing uses by default on Linux can get a bit wonky. Depending on the GTK theme you are using, menubars can actually become unusable!
You can change the Look and Feel to something that works by editing the file:
Be sure that Processing is not running when you do this. To use the standard Swing Metal Look and Feel, change the lines that read:
Metal may not be pretty, but it seems to be pretty robust!
Note that because Processing uses its own JRE, it will ignore LnF and other settings you may have configured for your default JRE.