Yep, even make SUBTARGET=mess DEBUG=1 REGENIE=1 after a make clean creates a binary, started with './mess64d -debug -window' that will freeze the Ubuntu 14.04 window system when quitting from the debugger in one of my known working driver sessions chosen from the new internal UI. I even had to chicken out via a virtual terminal by Ctl-Alt-Del.

Is it the new UI on Ubuntu that I should avoid and how so? (I just realized that this is probably the wrong thread, sorry...)

Because I can