Agreed. I always think I’ll one day build my own UI framework. Drawing to the screen and computing layouts is manageable, but then I think about the details: widget focus, text selection, platform keyboard shortcuts, text rendering (which encompasses things like right-to-left text rendering, ligatures, and a million other hateful complexities), etc and I realize why there are so few toolkits out there.

I do think it would be interesting to define a minimal set of user interface conventions that would forego a lot of features but would allow developers to stand a fighting chance of building a UI toolkit. If it became popular enough, users could come to understand its conventions even if they aren’t platform native (not ideal, but something has to change).