Github CI has a mac os runner which is headless AFAIK, but it has a software renderer. However, glfw can't get to it - this can be illustrated by this pull (do not merge) which runs glfwinfo after build:
#2569

Both OSX OpenGL (the GLUT part) and SDL2 can get at the apple software renderer within github CI, so this is a glfw specific problem. It would be useful to nake this work, to allow glfw to be used within github's own CI workflow for testing.

I came to be here after going down the rabbit hole of trying to look at a mac os specific problem in skia-python (which has CI testing for all three platforms, and been using glut for testing on mac os x for the above reasons, instead of glfw as in the case of linux with Xvfb):
kyamagu/skia-python#214
and downstream FlorianRhiem/pyGLFW#80