When clicking a song while another was playing, PLAYER_SYNC_QUEUE and
PLAYER_CURRENT would fire before the music player switched tracks,
wiping the playIndex set by PLAYER_PLAY_TRACKS. This caused the player
to stay on the old track instead of switching to the clicked one.
Now reduceSyncQueue and reduceCurrent preserve a pending playIndex until
the music player confirms it actually reached the requested track.
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