Function reconcileSinglePortal

packages/react-reconciler/src/ReactChildFiber.js:1728–1762

(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    portal: ReactPortal,
    lanes: Lanes,
  )

  }

  function reconcileSinglePortal(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    portal: ReactPortal,
    lanes: Lanes,
  ): Fiber {
    const key = portal.key;
    let child = currentFirstChild;
    while (child !== null) {
      // TODO: If key === null and child.key === null, then this only applies to
      // the first item in the list.
      if (child.key === key) {
        if (
          child.tag === HostPortal &&
          child.stateNode.containerInfo === portal.containerInfo &&
          child.stateNode.implementation === portal.implementation
        ) {
          deleteRemainingChildren(returnFiber, child.sibling);
          const existing = useFiber(child, portal.children || []);
          existing.return = returnFiber;
          return existing;
        } else {
          deleteRemainingChildren(returnFiber, child);
          break;
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }

    const created = createFiberFromPortal(portal, returnFiber.mode, lanes);
    created.return = returnFiber;
    return created;
  }

  // This API will tag the children with the side-effect of the reconciliation
  // itself. They will be added to the side-effect list as we pass through the