Occurs in Fiber with multiple instances of the React package (which can't share current owner).
}
    if (oldFiber === null) {
      // 
      // 
      for (; !step.done; newIdx++, (step = newChildren.next())) {
        const newFiber = createChild(returnFiber, step.value, priority);
        if (newFiber === null) {
          continue;
        }
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        if (previousNewFiber === null) {
          // 
          resultingFirstChild = newFiber;
        } else {
          previousNewFiber.sibling = newFiber;
        }
        previousNewFiber = newFiber;
      }
      return resultingFirstChild;
    }
    // 
    const existingChildren = mapRemainingChildren(returnFiber, oldFiber);
    // 
    for (; !step.done; newIdx++, (step = newChildren.next())) {
      const newFiber = updateFromMap(
        existingChildren,
        returnFiber,
        newIdx,
        step.value,
        priority,