This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress.child = mountChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderLanes,
    );
  } else {
    
    
    
    
    
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      current.child,
      nextChildren,
      renderLanes,
    );
  }
}
function forceUnmountCurrentAndReconcile(
  current: Fiber,