Occurs in Fiber with multiple instances of the React package (which can't share current owner).
newFiber: Fiber,
    lastPlacedIndex: number,
    newIndex: number,
  ): number {
    newFiber.index = newIndex;
    if (!shouldTrackSideEffects) {
      
      return lastPlacedIndex;
    }
    const current = newFiber.alternate;
    if (current !== null) {
      const oldIndex = current.index;
      if (oldIndex < lastPlacedIndex) {
        
        newFiber.effectTag = Placement;
        return lastPlacedIndex;
      } else {
        
        return oldIndex;
      }
    } else {
      
      newFiber.effectTag = Placement;
      return lastPlacedIndex;
    }
  }
  function placeSingleChild(newFiber: Fiber): Fiber {