The Fast Light ToolKit ("FLTK", pronounced "fulltick") is a LGPL'd C++
graphical user interface for X11.

FLTK provides modern GUI functionality without the bloat and supports
3D graphics via OpenGL and its built-in GLUT emulation.

FLTK is designed to be small and modular enough to be statically linked,
but works fine as a shared library.  FLTK also includes an excellent UI
builder called FLUID that can be used to create applications in minutes.

WWW: https://www.fltk.org/