Nano-X is ideal for embedded systems on OpenBSD Microwindows or Nano-X is a small graphical windowing system that implements both Win32 and Nano-X (X11-like) APIs for clipped graphics drawing in windows on Linux, Mac OS X, EMSCRIPTEN, Android and other platforms. It is Open Source and licenced under the the Mozilla Public License. For creating GUIs, the Nuklear immediate mode GUI, Win32 builtin controls, and TinyWidget's controls based on Nano-X are included. FLTK can be used with the X11 compability library NX11.
https://github.com/ghaerr/microwindows
