can't get enter_notify_event to work

["Karl H. Beckers" <karl h beckers gmx net>]

Hi all,
can't get this to work the way I need it to and appreciate all clever
ideas.
I have a very simple test code that works as desired, but it does not
work in another app. I'm doing:

void
on_gtk_frame_right_enter_notify_event (GtkWidget * w, gpointer data) {
    printf("entered . \n");
}

        gtk_frame_right = gtk_window_new (GTK_WINDOW_TOPLEVEL);

        g_assert (gtk_frame_right);

        gtk_widget_set_size_request (GTK_WIDGET (gtk_frame_right),
                                     FRAME_WIDTH, pheight);
        gtk_widget_set_sensitive (GTK_WIDGET (gtk_frame_right), FALSE);
        gtk_window_set_title (GTK_WINDOW (gtk_frame_right),
"gtk_frame_right");
//        GTK_WINDOW (gtk_frame_right)->type = GTK_WINDOW_POPUP;
        gtk_window_set_resizable (GTK_WINDOW (gtk_frame_right), FALSE);
        colormap = gtk_widget_get_colormap (gtk_frame_right);
        r_eb = gtk_event_box_new ();
        gtk_container_add (GTK_CONTAINER (gtk_frame_right), r_eb);
        gtk_widget_show(r_eb);


...

        g_signal_connect (G_OBJECT(toplevel), "configure-event",
                          G_CALLBACK (on_gtk_frame_configure_event),
NULL);

        g_signal_connect (G_OBJECT(r_eb), "enter_notify_event",
                          G_CALLBACK
(on_gtk_frame_right_enter_notify_event), NULL);


the callback is never fired. I've also tried to add
GDK_ENTER_NOTIFY_MASK explicitly to no betterment.

Is there anything special to watch out for here?

TIA,

Karl.