This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress,
  primaryChildren,
  renderLanes,
) {
  const mode = workInProgress.mode;
  const primaryChildProps: OffscreenProps = {
    mode: 'visible',
    children: primaryChildren,
  };
  const primaryChildFragment = createFiberFromOffscreen(
    primaryChildProps,
    mode,
    renderLanes,
    null,
  );
  primaryChildFragment.return = workInProgress;
  workInProgress.child = primaryChildFragment;
  return primaryChildFragment;
}
function mountSuspenseFallbackChildren(