The poll loop's seamless-transition branch guessed whether a track-id change
while still playing was a gapless hand-off via a peek_upcoming(1) == track_id
heuristic. When that heuristic missed (queue pointer lagging the engine), the
meta was never refreshed: the seek bar kept advancing while the title/art/album
stayed on the previous track (the reported now-playing populate bug).
Replace the heuristic with reconciliation to what is ACTUALLY playing. New
frontend-agnostic primitive QueueManager::sync_current_to_id(id) moves the queue
pointer to the track the engine advanced to (recording history + keeping the
shuffle cursor aligned) and reports whether it moved; QbzCore wraps it and emits
a queue update only on a real move. The poll loop now syncs to the live track_id
and always refreshes the card, doing the record/sidebar/prefetch bookkeeping only
on a genuine advance (a manual play already moved the pointer, so it is idempotent).
cc
vicrodh
c763d5b2fe