GDBusConnection *connection, gpointer data);
  static void on_proxy_created (GDBusProxy *proxy, GAsyncResult *result,
          gpointer data);
 +static void on_vertical_scroll(GDBusConnection *connection,
 +        const char *sender_name, const char *object_path,
 +        const char *interface_name, const char *signal_name,
 +        GVariant *parameters, Client *c);
  static void dbus_call(Client *c, const char *method, GVariant *param,
          GAsyncReadyCallback callback);
+ static GVariant *dbus_call_sync(Client *c, const char *method, GVariant
+         *param);
  static void on_web_extension_page_created(GDBusConnection *connection,
          const char *sender_name, const char *object_path,
          const char *interface_name, const char *signal_name,