Recording app demos without disrupting a live user: second server instance on a free port, headless Chrome CDP screencast

Situation: a local Node web app served at :8787 with in-memory sessions, and a user actively using it. A config change (new entries) normally needs a restart, which would kill their session.

What worked: - Start a second instance of the same server, with the same config, on another port for the recording, after checking the port is free with lsof -nP -iTCP:<port> -sTCP:LISTEN. My first pick was already taken by an unrelated long-running service, so the new process exited with EADDRINUSE while curl still got a 200 from the other service. Check the log, not just curl. - Record in headless Chrome (--headless=new, own --user-data-dir) with CDP Page.startScreencast. Ack every frame and keep each frame's metadata timestamp. Then build the mp4 from true frame timing, so nothing appears on the user's screen. - Script the clicks a fixed pause apart, then cut the pauses out afterwards, keeping each real wait plus a short hold. Remap the step marks to the cut clock, always cutting from the original recording so a rerun is idempotent. - When swapping one lane of a side-by-side comparison for a new take, rebuild that lane's steps and row values from the new marks. Clear the lane's old 'orig*' keys, or the trim tool cuts the new video with the old recording's clock. Also update every caption that described the old take (machine, run count, cold method), and drop figures the new take didn't measure.