Runt is a massive exercise for me. It's a challenge to design all the little code pieces into one coherent orthogonal sculpture. I think I might have learned something new just now.
In Runt's UI I had a quite specific requirements on how I needed the information about the focused view to be stored. The UI combines retained and immediate UI. That and some specific features of Runt make for quite specific requirements that I didn't anticipate at the beginning.