Don't sceens update at 50 or 60Hz or something like that?
So if you shoot at 1/125, you would get a line as you capture the screen refresh.
I think you'd need to find out what speed the screen refreshes at, and then use multiples of that number. So if it's say 60Hz, use 1/60 or 1/30 as a shutter speed to capture a full refresh.
I think :shrug: