This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
if (!includesSomeLane(renderLanes, workInProgress.childLanes)) {
    
    
    
    if (enableLazyContextPropagation && current !== null) {
      
      
      lazilyPropagateParentContextChanges(current, workInProgress, renderLanes);
      if (!includesSomeLane(renderLanes, workInProgress.childLanes)) {
        return null;
      }
    } else {
      return null;
    }
  }
  
  
  cloneChildFibers(current, workInProgress);
  return workInProgress.child;
}
function remountFiber(
  current: Fiber,
  oldWorkInProgress: Fiber,
  newWorkInProgress: Fiber,
): Fiber | null {
  if (__DEV__) {
    const returnFiber = oldWorkInProgress.return;
    if (returnFiber === null) {
      throw new Error('Cannot swap the root fiber.');
    }