There seems to be some people writing UI applications with the Godot game engine. Given the dismal state of desktop UI frameworks/tools, this seem to make more and more sense (very light binaries, solid cross-platform support, a WYSIWYG toolkit with good scripting functionality, a high-performance 2D renderer, access to C/C++ if you wan to go low-level, you name it.)
Examples:
https://www.wonderdraft.net/
https://github.com/Orama-Interactive/Pixelorama