usePersistPlaybackPosition

Remarks

time-update fires several times a second, so those writes are debounced at TIME_UPDATE_DEBOUNCE_MS. pause, seeking, and ended write immediately — they are the moments a learner is most likely to leave, and losing a debounce window of progress there is exactly what they would notice; those three also skip the server write window. The pending write is flushed on unmount, and on pagehide it leaves by navigator.sendBeacon, so a route change or a closed tab costs nothing.

Nothing is written before PlaybackPositionPersistence.openWriteGate is called. A cold load fires time-update at 0, and writing that would destroy the saved position before the learner is ever offered it.

Range and finiteness are not re-checked here — the PlaybackPosition value object owns those, and a second copy of the rules would be one to keep in sync. A detached player reporting NaN is silently dropped there.