This also tries to use save_under and/or hardware overlay planes. If these are not found, the default visual is used, and it relies on save_under for getting the menus to work. If neither overlay planes or save_under are available, there will be some small errors in the display where menus overlap.
If you get the SGI overlay planes your drawing is limited to three colors: FL_BLACK (0), FL_WHITE (7), and FL_LIGHT1 (49). All other colors will come out one of these three.
Fl_Menu_Window(int X, int Y, int W, int H, const char *l);