Occurs in Fiber with multiple instances of the React package (which can't share current owner).
}
      if (shouldTrackSideEffects) {
        if (oldFiber && newFiber.alternate === null) {
          // 
          // 
          deleteChild(returnFiber, oldFiber);
        }
      }
      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
      if (previousNewFiber === null) {
        // 
        resultingFirstChild = newFiber;
      } else {
        // 
        // 
        // 
        // 
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
      oldFiber = nextOldFiber;
    }
    if (newIdx === newChildren.length) {
      // 
      deleteRemainingChildren(returnFiber, oldFiber);
      return resultingFirstChild;
    }
    if (oldFiber === null) {
      // 
      //