This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress.child !== currentPrimaryChildFragment
  ) {
    const progressedPrimaryFragment: Fiber = (workInProgress.child: any);
    primaryChildFragment = progressedPrimaryFragment;
    primaryChildFragment.childLanes = NoLanes;
    primaryChildFragment.pendingProps = primaryChildProps;
    if (enableProfilerTimer && workInProgress.mode & ProfileMode) {
      
      
      
      
      primaryChildFragment.actualDuration = 0;
      primaryChildFragment.actualStartTime = -1;
      primaryChildFragment.selfBaseDuration =
        currentPrimaryChildFragment.selfBaseDuration;
      primaryChildFragment.treeBaseDuration =
        currentPrimaryChildFragment.treeBaseDuration;
    }
    
    
    
    workInProgress.deletions = null;
  } else {
    primaryChildFragment = createWorkInProgressOffscreenFiber(
      currentPrimaryChildFragment,
      primaryChildProps,
    );