See #17798
const id = getFiberID(primaryFiber);
    if (isRoot) {
       R
       S
      pendingUnmountedRootID = id;
    } else if (!shouldFilterFiber(fiber)) {
       T
       w
       a
      if (isSimulated) {
        pendingSimulatedUnmountedIDs.push(id);
      } else {
        pendingRealUnmountedIDs.push(id);
      }
    }
    fiberToIDMap.delete(primaryFiber);
    idToFiberMap.delete(id);
    primaryFibers.delete(primaryFiber);
    const isProfilingSupported = fiber.hasOwnProperty('treeBaseDuration');
    if (isProfilingSupported) {
      idToRootMap.delete(id);
      idToTreeBaseDurationMap.delete(id);
    }
  }
  function mountFiberRecursively(
    fiber: Fiber,
    parentFiber: Fiber | null,
    traverseSiblings: boolean,
    traceNearestHostComponentUpdate: boolean,
  ) {