public void when_client_message (ClientMessage event) {
    // client asks to change window state from normal to iconic 
    Client client = (Client) Client.intern (display, event.window_id);
    if (client.early_unmapped || client.early_destroyed) return;

    Atom type = event.type ();
    if (event.format () == 32 
      && type.name.equals ("WM_CHANGE_STATE")
      && event.wm_data () == Window.WMHints.ICONIC) {

      hide (client);

    } else
      alert_user ("Unhandled client message: " + type);
  }