an respond if the user
      // or UA ends the session for any reason.
      session.addEventListener('end', onSessionEnded);
      session.addEventListener('inputsourceschange', onInputSourcesChange);
      // trackedSources are still experimental. Don't rely on this feature yet.
      session.addEventListener('trackedsourceschange', onInputSourcesChange);

      session.addEventListener('visibilitychange', e => {
        // remove hand controller while blurred
        if(e.session.visibilityState === 'vis