It is highly recommended that you put the following code before the first show() of any window in your program:
Fl::visual(FL_DOUBLE|FL_INDEX)
This makes sure you can use Xdbe on servers such as Irix where double buffering does not exist for every visual.
Fl_Double_Window::Fl_Double_Window(int W, int H, const char *l=0);
Fl_Double_Window::Fl_Double_Window(int X, int Y, int W, int H, const char *l);
ulong Fl_Double_Window::pixmap() const;