Repository: facebook/react
File: packages/react-server/src/ReactFizzServer.js (JavaScript)

Code before:
      segment.id = boundary.rootSegmentID;
      // We postponed directly inside the Suspense boundary so we mark this for resuming.
      const boundaryNode: ReplaySuspenseBoundary = [
        boundaryKeyPath[1],
        boundaryKeyPath[2],
        children,
        boundary.rootSegmentID,
        fallbackReplayNode,
        boundary.rootSegmentID,
      ];
      trackedPostpones.workingMap.set(boundaryKeyPath, boundaryNode);
      addToReplayParent(boundaryNode, boundaryKeyPath[0], trackedPostpones);
      return;
    } else {
      let boundaryNode: void | ReplayNode =
        trackedPostpones.workingMap.get(boundaryKeyPath);
      if (boundaryNode === undefined) {
        boundaryNode = [
          boundaryKeyPath[1],
          boundaryKeyPath[2],
          children,
          null,
          fallbackReplayNode,
          boundary.rootSegmentID,
        ];
        trackedPostpones.workingMap.set(boundaryKeyPath, boundaryNode);
        addToReplayParent(boundaryNode, boundaryKeyPath[0], trackedPostpones);
      } else {
        // Upgrade to ReplaySuspenseBoundary.
        const suspenseBoundary: ReplaySuspenseBoundary = (boundaryNode: any);
        suspenseBoundary[4] = fallbackReplayNode;
        suspenseBoundary[5] = boundary.rootSegmentID;
      }
      // Fall through to add the child node.
    }
  }

  // We know that this will leave a hole so we might as well assign an ID now.
  // We might have one already if we had a parent that gave us its ID.
  if (segment.id === -1) {
    if (segment.parentFlushed && boundary !== null) {
      // If this segment's parent was already flushed, it means we really just
      // skipped the parent and this segment is now the root.
      segment.id = boundary.rootSegmentID;
    } else {
      segment.id = request.nextSegmentId++;
    }
  }

  if (task.childIndex === -1) {
    // Resume starting from directly inside the previous parent element.

Review comment (suggestion):
There is a reference below to
```
throw new Error(
  'It should not be possible to possible to postpone at the root...
 )
```

This whole boundaryKeyPath existence check can go away or if you're concerned it could still happen in maybe we just make the message clear it isn't about postponing at the root

Code after:
  // only need to reset it to the previous value at the very end.
  task.treeContext = prevTreeContext;
  task.keyPath = prevKeyPath;
}

function trackPostpone(
  request: Request,
  trackedPostpones: PostponedHoles,
  task: Task,
  segment: Segment,
): void {
  segment.status = POSTPONED;

  const keyPath = task.keyPath;
  const boundary = task.blockedBoundary;

  if (boundary === null) {
    segment.id = request.nextSegmentId++;
    trackedPostpones.rootSlots = segment.id;
    if (request.completedRootSegment !== null) {
      // Postpone the root if this was a deeper segment.
      request.completedRootSegment.status = POSTPONED;
    }
    return;
  }

  if (boundary !== null && boundary.status === PENDING) {
    boundary.status = POSTPONED;
    // We need to eagerly assign it an ID because we'll need to refer to
    // it before flushing and we know that we can't inline it.
    boundary.rootSegmentID = request.nextSegmentId++;

    const boundaryKeyPath = boundary.trackedContentKeyPath;
    if (boundaryKeyPath === null) {
      throw new Error(
        'It should not be possible to postpone at the root. This is a bug in React.',
      );
    }

    const fallbackReplayNode = boundary.trackedFallbackNode;

    const children: Array<ReplayNode> = [];
    if (boundaryKeyPath === keyPath && task.childIndex === -1) {
      // Since we postponed directly in the Suspense boundary we can't have written anything
      // to its segment. Therefore this will end up becoming the root segment.
      segment.id = boundary.rootSegmentID;
      // We postponed directly inside the Suspense boundary so we mark this for resuming.
      const boundaryNode: ReplaySuspenseBoundary = [
        boundaryKeyPath[1],
        boundaryKeyPath[2],
        children,