This window type is used to make the pop-up windows used by the
Fl_Menu class. It tries to use hardware overlay planes if possible.
If so then drawing is very limited: only the colors
FL_BLACK (0), FL_WHITE (7), and FL_GRAY (47) work. All other come out
as one of these three. Your drawing code can tell it is in this mode
by checking the static variable char fl_overlay
.
Fl_Menu_Window::Fl_Menu_Window(int X, int Y, int W, int H,
const char *l);
Fl_Menu_Window::set_overlay();
Fl_Menu_Window::clear_overlay();
In case you need to draw in color (such as the menus do sometimes if labelcolor is set) you can force it not to use the overlay with clear_overlay();