When a background terminal() command backgrounds its own long-lived
child (`node server.js &`, `sleep 300 &`), the grandchild inherits the
write end of the reader thread's stdout pipe. The direct bash child
exits promptly, but the pipe never reaches EOF while the grandchild
lives — so `_reader_loop`'s blocking `read1()` parked the thread
forever, `session.exited` never flipped on its own, and
`notify_on_complete` was silently lost. `_reconcile_local_exit`
(#17327) only runs lazily from poll()/wait(), so nothing autonomous
ever surfaced the exit; each occurrence also leaked a reader thread
and pipe fd for the grandchild's lifetime.
Fix: on POSIX, drain via select() with a short poll interval and stop
shortly after the direct child exits even if the pipe hasn't EOF'd —
the same pattern the foreground path uses in
tools/environments/base.py::_wait_for_process (#8340). Windows pipes
don't support select(), so the blocking path is kept there with the
existing lazy reconcile as the safety net; mocked/iterator stdout
streams (no usable fileno) also keep the historical path.
Fixes#68915
9e4492fd74