_drain_queued_prompt attaches the transport pinned to the queued envelope, but the client that queued the prompt may have disconnected while the prompt sat in the queue. Attaching it then pins a dead peer into the session's fan-out, where it costs one failed write before the fan-out prunes it. That is not a regression — the line this replaced rebound the whole slot to that same dead transport, which cost the session for the entire drained turn rather than one write — but the guard is one condition, so it goes in.
_transport_is_dead is the predicate already used by the reaper and the orphan check: the parked drop sentinel, or a transport carrying _closed. Drain semantics are otherwise unchanged. The prompt still runs, and the clients already attached keep their stream; only the dead peer's attachment is skipped.
tests/tui_gateway/test_multi_client_fanout.py pins that the drained prompt still dispatches while the disconnected queuer stays out of the slot, alongside the existing case where a live queuer is attached and the single-client control where the queuer takes an unoccupied slot.
Raised by the automated review on #86784.
Ryan Tucker
4d32503f86