 any reason.
    // At this point the session object is no longer usable and should be
    // discarded.
    function onSessionEnded(event) {
      xrButton.setSession(null);

      // In this simple case discard the WebGL context too, since we're not
      // rendering anything else to the screen with it.
      renderer = null;
    }

    function onInputSourcesChange(event) {
      onSourcesChange(event, "input_");
    }

    function onTrackedSourcesChange(event) {
      onSourcesChange(event, "tracked_"