microui
A tiny immediate-mode UI library
...The project is intentionally small and gives developers room to add their own controls, rendering backends, and interaction patterns. It is especially useful for tools, games, editors, embedded interfaces, and experiments where portability and simplicity matter more than a complete widget toolkit. The library avoids unnecessary complexity and keeps its scope narrow so it can remain easy to understand, modify, and integrate. It is a strong fit for C projects that need basic UI behavior while leaving rendering and platform details under the developer’s control.