Just a note to myself, in case I ever need this again, and it may help someone else....
I have a boring 17" no-name cathode ray monitor at home, coupled to a Wintel box with a reasonably useful graphics chipset on the motherboard.
The chipset appears to know how to do a virtual screen or 3000 or so pixels square, and window the screen display onto it, not that I find this particularly useful, but anyway...
One of my children has a computer game that kicks the screen into a certain resolution, and when you quit, it resets the screen resolution back into this mega-enormous resolution, and the screen attempts to try and display it at 70-80 times a second and fails dismally, instead of just setting a viewport into it. I have no idea who's fault this is. The result is what Neal Stephenson might call a snowcrash.
Actually, it's not. The machine is still completely functional except that it looks like you're looking at encrypted TV without a decoder.
After a number of reboots, the machine finally gets the message and resets the resolution to something sane. No, Safe Mode doesn't work. When I return to normal mode after a Safe session, the screen remains scrambled.
So finally after having done this too many times, I realised was that all I needed to do was to note the keyboard shortcut sequence, and then I could land the beast on instruments only.
And here is the recipe:
Shift-Ctrl-Tab # select previous (i.e. last, right-most) display tab
At the point the screen realigns itself with reality again, and then I can go and choose some other resolution.