I agree that it's not well defined, but I'd guess that a good rule of thumb is "fd 0 isn't isatty, and the process has the info (from env vars or arguments) nec

I agree that it's not well defined, but I'd guess that a good rule of thumb is "fd 0 isn't isatty, and the process has the info (from env vars or arguments) necessary to open graphically" with an additional switch for overriding in either direction.