Occurs in Fiber with multiple instances of the React package (which can't share current owner).
returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    portal: ReactPortal,
    priority: PriorityLevel,
  ): Fiber {
    const key = portal.key;
    let child = currentFirstChild;
    while (child !== null) {
      // 
      // 
      if (child.key === key) {
        if (
          child.tag === HostPortal &&
          child.stateNode.containerInfo === portal.containerInfo &&
          child.stateNode.implementation === portal.implementation
        ) {
          deleteRemainingChildren(returnFiber, child.sibling);