When someone says "cross-platform native UI library", the first thing that comes to my mind is "Lazarus Component Library" (aka LCL).
I like its 4 features: 1. static linking 2. native 3. excellent OO abstraction 4. WYSIWYG I always wish there was an equivalent to Nim, or just translate it's free Pascal code to Nim. <https://en.wikipedia.org/wiki/Lazarus_Component_Library> <https://wiki.freepascal.org/LCL_Components> <https://wiki.freepascal.org/LCL>