What Event if Window is not visible anymore

[Steffen Raach <Steffen raach de>]

Hi,

I am trying to start a function whenever a (Popup-)Window gets (partly)
invisible. I have tried 

gtk_signal_connect(GTK_OBJECT(window),
                   "visibility_notify_event",
                   GTK_SIGNAL_FUNC(visibility_notify_event_cb),
                   NULL);

but I never get a call of:

static gint visibility_notify_event_cb(GtkWidget* window,
GdkEventVisibility* e, gpointer data)

Where is the problem?

Steffen