Prohibit rowcount and rowindex on rows in treegrid and prohibit posinset and setsize on rows in tables and grids

[mcking65]

w3c/aria-practices#1442 raises the question of how to set aria-rowcount and aria-rowindex in a treegrid where not all rows are in the DOM and suggests that aria-posinset and aria-setsize are all that is needed in a treegrid.

We should consider prohibiting aria-rowcount and aria-rowindex on rows in a treegrid. Using them instead of or in addition to aria-posinset and aria-setsize could create problems.

Similarly, if a row is in a grid or table, aria-setsize and aria-posinset should not be allowed.

[joanmarie]

If we go down this path, what would we do about:

1. aria-rowindex which is supported on gridcell too.
2. aria-rowcount which is supported on treegrid too.
3. aria-colindex which is supported on gridcell.
4. aria-colcount which is supported on treegrid.

[joanmarie]

Looking at #558 (comment), Aaron stated:

aria-rowcount is apparently for the total number of rows in the table, not on this level. So I think we still need setsize/posinset.

If that is the use case, then that sounds like there are cases in which these properties are not the same/redundant.

[smhigley]

Is the authoring guidance that someone making a virtualized treegrid needs to always set both to address both use cases? Or that they should choose one system based on how they're using treegrid?