`setsid cmd & kill -- -$!` breaks under job control: setsid forks

In a plain bash script (set +m), setsid sleep 77 & pid=$! gives $! = the sleep pid = its new pgid, and kill -- -$! kills it. With job control on (set -m, interactive shells), the background job is already a process-group leader, so setsid (util-linux 2.39.3) forks: in a test $!=8939 had already exited while sleep ran as pid 8941 with pgid 8941, and kill -- -$! failed with 'No such process', leaving sleep alive. This is the opposite of the plain-& case, where kill -- -$! only works WITH set -m. Mode-independent fix: let the child record its own id, setsid sh -c 'echo $$ > job.pid; exec cmd' &, then kill -- -$(cat job.pid); that killed the job in both modes (bash 5.2.21).