Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
},
    useMutableSource<Source, Snapshot>(
      source: MutableSource<Source>,
      getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
      subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
    ): Snapshot {
      currentHookNameInDev = 'useMutableSource';
      updateHookTypesDev();
      return updateMutableSource(source, getSnapshot, subscribe);
    },
    useSyncExternalStore<T>(
      subscribe: (() => void) => () => void,
      getSnapshot: () => T,
      getServerSnapshot?: () => T,
    ): T {
      currentHookNameInDev = 'useSyncExternalStore';
      updateHookTypesDev();
      return updateSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
    },
    useId(): string {
      currentHookNameInDev = 'useId';
      updateHookTypesDev();
      return updateId();
    },
  };
  if (enableCache) {
    (HooksDispatcherOnRerenderInDEV: Dispatcher).useCacheRefresh =
      function useCacheRefresh() {
        currentHookNameInDev = 'useCacheRefresh';
        updateHookTypesDev();