I agree that it's not well defined, but I'd guess that a good rule of thumb is "...

dllthomas on June 5, 2021 | parent | context | favorite | on: Ftxui: C++ Functional Terminal User Interface

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.