set_opacity problem (source attached, less than 12 lines)

["Anibal F. Martinez Cortina" <linuxkid zeuz gmail com>]

Hey list, this is just a simple "test dock-like application" I was trying
to do.
Whenever I use Gtk::Window::set_opacity, the transparency, if it's not
1(even 0.999999) will make the window entirely transparent.
I do realize this is not the correct way to get transparencies, but it's
just starting:
[code]
#include <gtkmm.h>
int main(int argc, char * argv[]) {
Gtk::Main kit(argc,argv);
Gtk::Window ventana;
ventana.set_default_size(600,100);
ventana.move(gdk_screen_width()/4,gdk_screen_height());
ventana.set_decorated(false);
ventana.set_opacity(0.9999);
Gtk::Main::run(ventana);
return EXIT_SUCCESS;
}
[/code]

PS: Yes, It's GtkMM and I'm starting, but since it's related, perhaps you
can help?
Thanks in advance.