I've now found how to add widgets to a GL3-using Fl_Gl_Window in a cross-platform way.

The attached program is an example that runs on all 4 platforms (Windows, X11, Wayland, macOS) with the last git code

of FLTK 1.4 where a partially transparent button is laid over a GL3 scene.

With those platforms that allow the so-called compatibility profile, the key is to call

glUseProgram(0);

between the end of the GL3  and the beginning of the GL2 drawing parts.

It has also been necessary to change the X11 driver code  to have it ask for a compatibility profile.

macOS does not provide the compatibility profile. The solution is to create an additional Fl_Gl_Window above the window

containing the GL3 scene, to put FLTK widgets therein, and to give it a transparent background.