CI diagnostics (tmux 3.4 on ubuntu-24.04) show the /exit hang is tmux's:
after /exit the pane process is a single-threaded zombie of the tmux
server (Threads:1, PPid = tmux server), the server is running with
SIGCHLD caught, not blocked and not pending, and pane_dead_status never
fills for 60s. The TUI had already exited (its epilogue is in the PTY
transcript). A zombie tmux has not reaped after 2s now gets a SIGCHLD
nudge so tmux runs its waitpid loop and reports the real exit status; a
process that is still running is never touched and still times out.
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