Logged as WJ-7338 and posted as resolved in 1.2.11 but still not working.
Yep. Sorry about that. The designers for the DGV columns and the TreeView have been changed a lot after that fix and ended up reverting it. Now the designers are live, meaning that when you edit columns or tree nodes the changes are reflected immediately in the designer.
Anyway, there is a refreshed build available with this fix refixed. 🙂