Comment 6 for bug 1734271

I have reproduced this bug on actual hardware (Dell XPS 15 9550) using a live USB of today's 18.04 daily ISO (2017-12-28). One sequence of events that reproduced the issue was:

1) Boot from USB (UEFI boot in this case)
2) Close Welcome application
3) Open MATE Tweak
4) Choose Cupertino panel layout
5) Choose Compiz compositor (this may or may not be necessary)
6) Open Caja, press Alt a few times to bring up the HUD
7) Open Terminal using CTRL-ALT-T shortcut
8) ----- Notice in upper right, extra "EN" keyboard layout icon next to keyboard indicator
9) Press Alt
10) X session freezes and becomes unresponsive

At this point the only way I've found to regain control is to switch to another TTY and sudo killall lightdm.