Yeah, I noticed the EverproTreeGrid today as well, just after finishing my experiment. Pity that it doesn’t have a demo. Apparently, some things in it could be improved, like the use of font icons instead of image file for the icon.
The approach seems to be a bit different; it extends Grid and uses a custom ButtonRenderer instead of ItemClickListener to handle expand/collapse clicks. It also sets the expanded/collapsed styles in the renderer instead of CellStyleGenerator. I can’t say if that’s really better than my purely server-side solution, as in any case the clicks need to be handled in the server-side as well, and the renderer requires a bit unnecessary client-side code. For rendering the styles, the renderer could be more light-weight than my CellStyleGenerator solution, maybe. The add-on uses a custom container wrapper – that is probably a good approach for generalization – I was thinking of extending GeneratedPropertyContainer for the same purpose, but an entirely custom wrapper may be fine as well. It doesn’t seem to use container filtering features though, but some custom operations on the item sets. I didn’t look it very closely, but it’s interesting. It would be great if it proves to be a good way to have the hierarchical support for Grid.