When a session finishes streaming while the user has switched to a different
session, setBusy(false) was draining S.session.session_id (the currently
*viewed* session) instead of the session that actually finished. Queued
follow-up messages were silently dropped.
Root cause: setBusy() has no context about which session triggered it.
The activeSid closure variable inside attachLiveStream() knew the right
session but was not propagated.
Fix: add _queueDrainSid module global (null by default). Stream done and
error handlers set it to activeSid immediately before calling setBusy(false).
setBusy(false) reads and clears _queueDrainSid, falling back to S.session if
it is unset (the common case where the user hasn't switched away).
Handlers patched: done event, start-call error handler, stream_end/stream_stop
reconnection fallback, and max-retry error exit.
Co-authored with Claude Sonnet 4.6 / Anthropic.
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