Comment 14 for bug 336721

I've done the same thing for GTK/GDK. Used the basic 'Hello GTK' source from

http://library.gnome.org/devel/gtk-tutorial/stable/c39.html#SEC-HELLOWORLD

and amended it to get the screen number:

    gtk_init (&argc, &argv);

    /* create a new window */
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    screen = gtk_window_get_screen(GTK_WINDOW (window));
    screen_num = gdk_screen_get_number(screen);

Again, this works correctly on both screens, reporting the correct screen number.

This is starting to narrow down the issue towards gnome-panel (I see launcher has a launcher_get_screen() function) or maybe gnome-session ?