Spent some time playing with Tkinter, and building a real desktop app with it. It’s pretty specific to my needs, but is open to customization by others.
Building UIs with Tkinter was interesting (not frustrating), and it feels almost-there-but-not-quite-there. I still think that HTML/CSS/JS is the best API out there for UI (the good parts), but Tkinter’s mental model and API is quite nice too. Fairly intuitive.