A session can exit and run onExit (which deletes c.terms[id]) in the gap
between subscribe returning exited=false and openTerminal assigning
c.terms[id]. The delete is a no-op there since the key isn't set yet, so
the later assign resurrects a stale entry for a dead pane, trapping every
future open for that id on the connection. Re-apply the delete after the
assign when onExit fired in the window, tracked by a c.mu-guarded flag.
Add a stress regression test that races the exit against the assign.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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