In an earlier PR (#17259), I removed the rowindex and posinset attributes in grouped detailslist and groupedlist, because the index values within the passed-in data didn't work as ARIA index values for screen readers, and were causing problems.

This is ideal for grids and treegrids that are not virtualized, but doesn't support ones that are, so I'm opening this issue to track fixing explicit ARIA index/count values for virtualized Grouped DetailsList and GroupedList uses.

Related #17423 (the DetailsRow attribute added in that PR should make this possible)