This is a minimal-state, immediate-mode graphical user interface toolkit written
in ANSI C and licensed under public domain. It was designed as a simple
embeddable user interface for application and does not have any dependencies, a
default render backend or OS window/input handling but instead provides a highly
modular, library-based approach, with simple input state for input and draw
commands describing primitive shapes as output. So instead of providing a
layered library that tries to abstract over a number of platform and render
backends, it focuses only on the actual UI.

WWW: https://github.com/Immediate-Mode-UI/Nuklear