useCompleteWhenWatched

Remarks

The finish rule itself lives in hasFinishedWatching; this hook is the part that has to know when it is fair to act on it. Two refs carry that:

• A gate, closed until playback begins. Opening a lesson whose stored position is already past the threshold fires time-update at that position, and writing then would record a completion the learner did not earn on this visit. The lesson already reads as complete from its position, so nothing is lost by waiting.
• A "written" flag. time-update fires several times a second, and the last fifteen seconds of a lesson would otherwise write the same mark fifty times.

The write goes through markLessonComplete — the same path the manual button uses — so there is one stored notion of "done" rather than two. The same "written" flag makes the celebration fire once: the burst belongs to the moment the lesson is finished, not to every progress event after it. Completion is never withdrawn here: rewinding a finished lesson leaves the mark standing.

The lesson's own durationSeconds is preferred, and the provider's reported duration is the fallback, so a lesson whose metadata is missing a runtime can still be finished.