returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    element: ReactElement,
    expirationTime: ExpirationTime,
  ): Fiber {
    const key = element.key;
    let child = currentFirstChild;
    while (child !== null) {
      // 
      // 
      if (child.key === key) {
        if (
          child.tag === Fragment
            ? element.type === REACT_FRAGMENT_TYPE
            : child.type === element.type
        ) {