True once the last measure to render has been rendered -- no more batches remain. The measures to render are the ones render() draws: the whole sheet, or the draw range (drawFromMeasureNumber, drawUpToMeasureNumber), up to the last system or page drawn (drawUpToSystemNumber, drawUpToPageNumber).
The last measure position rendered so far (highest measure index): all its GraphicalMeasures, one per
staff/instrument (e.g. 3 for a voice + piano score). Empty if nothing has been rendered yet. They
share one source measure -- reach it and its number/index via any element's .parentSourceMeasure and
.parentSourceMeasure.measureListIndex (0-based).
The next measure position not yet rendered (the frontier): all its GraphicalMeasures (one per staff),
or empty once done. Same .parentSourceMeasure / .parentSourceMeasure.measureListIndex accessors.
Visual measures rendered so far, cumulative across batches.
Total visual measures to render: those of the sheet, or of the draw range (see done). With drawUpToSystemNumber or
drawUpToPageNumber, the measures after the last system or page drawn count until a batch's layout reaches it: where it
ends depends on the layout. On a single horizontal staffline, the first batch lays out all of them.
Progress returned by OpenSheetMusicDisplay.renderNext. Measure counts are visual (a multi-rest counts as one).