The replay is synchronized because the transport says so

The change window that broke production was recorded: ffmpeg against the operator's display, piped to an archive file as it happened. Now eight engineers need to watch it together. cat the webm back into a pipe with ?n=8; everyone opens the ?video URL.

Playback is synchronized by construction. The pipe writes each chunk to all eight receivers before reading the next, so nobody is thirty seconds ahead asking about a frame the others haven't reached. The slowest connection paces the room, the way a shared screen would: minus the meeting, the platform, and somebody's webcam software recompressing the evidence.

Watch-together is usually an app feature. Here it's a transport property: the sync isn't coordinated, it's unavoidable.

Nobody asks what second you're at; the transport already decided.