[mhd]

If I remember correctly, both FLTK and XForms have a commmon "ancestor" in the Forms library for SGI (pre-OpenGL days).

[p_l]

FLTK history explicitly mentions rewriting from libGL heavy code to pure X11 because of the differences in how useful GL was outside Xsgi[1]

[1] a quick example is how IRIX vector icons are actually serialised IRIX GL display lists