The Fast Light Tool Kit is a cross-platform C++ GUI toolkit for UNIX/Linux
(X11 or Wayland), Microsoft Windows and macOS.
FLTK provides modern GUI functionality without bloat and supports 3D graphics
via OpenGL and its built-in GLUT emulation.
FLTK also includes FLUID, an interactive user interface builder program.

From the FLTK website:

  FLTK 1.4 is a big step forward, adding new widgets and support of Wayland
  on Linux and FreeBSD systems.
  Work on FLTK 1.4 began 2016 because it was necessary to break the FLTK 1.3
  ABI in order to fix some long outstanding problems and to add new features.
  Resetting the ABI also allowed the removal of 200+ areas of conditional
  compilation involving ABI guards, greatly simplifying the code.
  FLTK 1.4 boasts a completely new abstract device driver class hierarchy
  to isolate platform specific code in one place instead of using #ifdef
  everywhere, and to make it easier to port to new platforms.

Homepage:
https://www.fltk.org/
