Description
POSIX requires that the job-control shell suspend itself if invoked in the background:
If its process group is not the foreground process group associated with the terminal (which would result from it being started by a job-control shell as a background job), the shell shall either stop itself by sending itself a SIGTTIN signal or, if interactive, attempt to read from standard input (which generates a SIGTTIN signal if standard input is the controlling terminal). If it is stopped, then when it continues execution (after receiving a SIGCONT signal) it shall repeat these steps.
Yash suspends itself by calling tcsetpgrp instead of raising SIGTTIN, and we're not going to change the behavior (see magicant/yash-rs#421 (comment)). We should document the intentional deviation from the standard.