treeitem scope should allow group with accessibility parent tree

[pkra]

Extracted from #2010 (comment)

We currently only have group with a treeitem parent but the group can have a tree parent as well.

[mcking65]

@pkra

Didn't you mean the opposite? the current editor's draft allows group as a child of tree, but that is inconsistent with the definition of treeitem. I submitted #2094 to fix this.

Trees do not have a concept similar to optgroup. If you want to group treeitems, they need a parent treeitem.

All the accessibility children of a tree need to be either parent treeitems or end node treeitems.

Even if a tree is partially virtualized, it seems to me that the rendered nodes should have their hierarchy represented in the DOM. If it were not, the tree would be inoperable.

I was inspired to investigate this by @WilcoFiers moment of confusion described in w3c/aria-practices#2887.

[pkra]

@mcking65 I was primarily concerned with the inconsistency. I have no strong opinion on how to resolve that inconsistency.

[MarioBatusic]

In my opinion, the only problem in the spec 1.3 is, that the tree role allows group role additionally to the treeitem role. What schould this group represent at all?
All other stuff is OK and correct - and works now for years.