static void
fullscreen_button_on_is_topmost_changed (GObject *object G_GNUC_UNUSED,
		                                 GParamSpec *property G_GNUC_UNUSED,
		                                 gpointer data)
{
	HeFullscreenButton *self = HE_FULLSCREEN_BUTTON (data);

	if (hildon_window_get_is_topmost (HILDON_WINDOW(self->parent_window))) {
		if (gdk_window_get_state (GTK_WIDGET (self->parent_window)->window) & GDK_WINDOW_STATE_FULLSCREEN) {
			fullscreen_button_enable (self);
		}
	}
	else {
		fullscreen_button_disable (self);
	}
}

[Before return self in he_fullscreen_button_new()]
    if (HILDON_IS_WINDOW(parent_window)) {
        g_signal_connect (parent_window, "notify::is-topmost",
            G_CALLBACK(fullscreen_button_on_is_topmost_changed), self);
    }

[After g_signal_handlers_disconnect_by_func (self->parent_window, fullscreen_button_destroy, self);]
g_signal_handlers_disconnect_by_func (self->parent_window, fullscreen_button_on_is_topmost_changed, self);