[dottrap]

    Honestly I kind of wonder why nobody has written a library that acts as a thin layer around and abstraction of native UI toolkits (AppKit, .NET, GTK+, etc) instead of a monolithic do-most-things-or-everything-yourself

IUP is exactly this. Native GUI focused and nothing more to be thin and lightweight. It's small (under 800K). It's written in pure C so language bindings can be made for any language. It is funded by the same university that created Lua. Liberal MIT license.

http://webserver2.tecgraf.puc-rio.br/iup/

Its weakness is it still lacks a native OS X backend. (People use GTK on Mac as a workaround.) There was an abandoned port attempt in 2010. Somebody just started trying to port it again a couple of weeks ago. (I'm sure additional volunteers/help are welcome.)

https://www.mail-archive.com/[e-mail]...

https://github.com/ewmailing/IupCocoa/tree/Cocoa