At the moment, the CollapsedGroups  collection in the grid state is the indexes of the root-level groups that you can collapse. I want to also be able to control the expanded state of nested groups.

ADMIN EDIT:

Ideas I can see are the following, do add your vote and comments on how you expect that to be exposed:

• a form of Id/field in the group descriptor that controls this. Caveat - groups don't generally have such an id and the descriptor does not control the UI, it controls the data.
• an event that provides the level, field name and group value that lets me choose this for every group as it renders. Caveat - might not plug in the state, but if you have a certain business logic for that you can just keep it in that event always, regardless of the grid state, or keep additional information in an application state.
• the current indexes could become recursive (see the image attached below). Caveat: you can't be sure that the next time data loads the data and so the groups will be the same when loading state