SubtitleDecoder

Turns subtitle packets into cues. The engine drains it on the actor between packets and flushes it on every seek; see SubtitleDecoderFactory.

Once a track's packets have run out at the end of its stream, the engine sends null, once, and receives what that gives: a decoder that holds its last cue until the next packet, as a closed caption decoder holds the caption on screen, gives it then. A decoder that refuses the null packet is offered it again after its output is received, a few times at most. The next packet after a drain comes only after a flush.