I don't have a complete solution, but I do have some ideas:
- Have you tried hooking it up to an external monitor? Sometimes auto-config can help you recover from weird states if you plug in a different display.
- On my Deck, I can reach a terminal by using CTRL-ALT-F4 that is separate from the main desktop mode (CTRL-ALT-F1 switches back). Default user seems to be called "deck". You may need to set a password to use sudo. I am not sure exactly how the desktop environment is set up on the Deck so I am not sure exactly what you need to change or where the files would be -- maybe check under /usr/share/X11/xorg.conf.d to see if anything is set to an insane value there?
- You might try sending Valve a support request
- As a last resort, you could try a factory reset. You'll nuke everything else on it too though...