I use i3wm, and here's what I've put in my config file:
exec_always --no-startup-id xrandr --output HDMI2 --left-of LVDS1
The problem is, aftertrying SuperTux and changing the resolution, it changed
for the whole computer.
I remember managing to change it back in the game.
But now the same thing happened while trying openxcom (non-free assets, but
the rest is), yet I can't change it back. Unfortunately I can't even remember
how I did it since before that I changed the in-game resolution so many times
without trouble.
Of course, I could restart the computer. I started to refresh my i3config
file, but no success.
I also remember having a screen assignment change when on and off the
Ultrabase (it's a Thinkpad), which I didn't solve yet. But if I can reset the
xrandr display without trouble, this might solve half of the latter issue,
and the whole of the first one.
Alternatively, can I script a specific resolution for when the game is turned
on, and when off, back to the latter config? Then I'll be able to play full
screen, full speed, no CPU screams, and keep my standard screen resolution
when not playing.
