class Fl_Menu_Window : public Fl_Single_Window

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);

Constructor.

Fl_Menu_Window::set_overlay();
Fl_Menu_Window::clear_overlay();

This flag tells fltk to try to use hardware overlay planes. By default it is on. Turning it off makes the Fl_Menu_Window act like a normal Fl_Window.

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();