Using gdk_window_get_device_position() gnome

  GdkDeviceManager *device_manager;
  GdkDevice *pointer;
  gint px, py;
  g_signal_emit (scroll_area, signals[VIEWPORT_CHANGED], 0,
                 new_viewport, old_viewport);

  if (scroll_area->priv->input_window == NULL)
    return;

  device_manager = gdk_display_get_device_manager (gtk_widget_get_display (GDK_WIDGET(GTK_WINDOW(scroll_area->priv->input_window))));
  pointer = gdk_device_manager_get_client_pointer (device_manager);
  gdk_window_get_device_position (GDK_WINDOW(scroll_area->priv->input_window), pointer, &px, &py, NULL);