This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
primaryChildFragment.selfBaseDuration = 0;
      primaryChildFragment.treeBaseDuration = 0;
    }
    fallbackChildFragment = createFiberFromFragment(
      fallbackChildren,
      mode,
      renderLanes,
      null,
    );
  } else {
    primaryChildFragment = createFiberFromOffscreen(
      primaryChildProps,
      mode,
      NoLanes,
      null,
    );
    fallbackChildFragment = createFiberFromFragment(
      fallbackChildren,
      mode,
      renderLanes,
      null,
    );
  }
  primaryChildFragment.return = workInProgress;
  fallbackChildFragment.return = workInProgress;
  primaryChildFragment.sibling = fallbackChildFragment;
  workInProgress.child = primaryChildFragment;
  return fallbackChildFragment;
}
function createWorkInProgressOffscreenFiber(