This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress,
  primaryChildren,
  fallbackChildren,
  renderLanes,
) {
  const mode = workInProgress.mode;
  const progressedPrimaryFragment: Fiber | null = workInProgress.child;
  const primaryChildProps: OffscreenProps = {
    mode: 'hidden',
    children: primaryChildren,
  };
  let primaryChildFragment;
  let fallbackChildFragment;
  if (
    (mode & ConcurrentMode) === NoMode &&
    progressedPrimaryFragment !== null
  ) {
    
    
    primaryChildFragment = progressedPrimaryFragment;
    primaryChildFragment.childLanes = NoLanes;
    primaryChildFragment.pendingProps = primaryChildProps;
    if (enableProfilerTimer && workInProgress.mode & ProfileMode) {
      
      
      
      
      primaryChildFragment.actualDuration = 0;
      primaryChildFragment.actualStartTime = -1;