0-based bar index, counting the first full bar as 0. formatBarPosition renders it 1-based. A pickup sounds before that bar, so its notes report bar -1.
Quarter-note offset from the start of the bar — not the felt beat. In 6/8
the second felt beat is at beat: 1.5; barPositionToPulse converts
to felt-beat numbering.
A position expressed as a bar index and a quarter-note offset within the bar.