The demo had a standby laptop, and the URL didn't notice the swap

Demo day. The product runs on your laptop, exposed at the container's public URL through the tunnel: one WebSocket out, local port 3000 bound to a fixed container port, the audience on an HTTPS address that existed before you sat down.

Your laptop dies in minute four. The container keeps the binding for a sixty-second grace period, answering visitors with a 503 and Retry-After: 5. Your co-founder, on the second laptop, opens the same expose with takeover: true. The container hands the binding to the new session atomically; the old one gets a reset, the new one serves. The URL on the projector never changed.

When a hostname belongs to the client session that opened it, a dead laptop takes it along. Here it belongs to the container, and laptops are interchangeable.

The demo had a standby laptop, and the URL didn't notice the swap.