See #17798
const nextParent = parentA.return;
        if (nextParent !== null) {
          a = b = nextParent;
          continue;
        }
         If there's no parent, w
        break;
      }
       I
       a
       p
      if (parentA.child === parentB.child) {
        let child = parentA.child;
        while (child) {
          if (child === a) {
             W
            if (isFiberMountedImpl(parentA) !== MOUNTED) {
              throw Error('Unable to find node on an unmounted component.');
            }
            return fiber;
          }
          if (child === b) {
             W
            if (isFiberMountedImpl(parentA) !== MOUNTED) {
              throw Error('Unable to find node on an unmounted component.');
            }
            return alternate;
          }
          child = child.sibling;
        }