> I checked the docs and found gl_texture_pile_height, it is set by default to 100 so I lowered to 1. The problem is gone.
>
> I'm sure this affects performance so if the problem is a bug that is fixable that would be great.
>
> Any clues?
>
> Thanks.
> Daniel

A discussion about this has been started over on fltk.dev to try and get the lib devs input:

http://www.fltk.org/newsgroups.php?gfltk.development+v:12457

Please can you post any notes and observations (such as this gl_texture_pile_height one) into that thread too, to make sure everyone is seeing it (not all the devs follow fltk.opengl you see...)