Function watch_decode_progress 

pub fn watch_decode_progress(
    progressed: impl Fn() + 'static,
) -> DecodeProgressWatch

Calls progressed on this thread each time a decoder operation running on it produces a frame, or a packet copy’s plan reads a source packet, until the guard drops.

An operation such as a seek can decode many frames before it returns, and a caller that watches decode for stalls from outside (a host’s watchdog) takes this as the operation being alive rather than blocked.