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`.
return updateRefresh();
      };
  }
  if (enableUseHook) {
    (HooksDispatcherOnRerenderInDEV: Dispatcher).use = use;
  }
  if (enableUseMemoCacheHook) {
    (HooksDispatcherOnRerenderInDEV: Dispatcher).useMemoCache = useMemoCache;
  }
  if (enableUseEffectEventHook) {
    (HooksDispatcherOnRerenderInDEV: Dispatcher).useEffectEvent =
      function useEffectEvent<Args, Return, F: (...Array<Args>) => Return>(
        callback: F,
      ): F {
        currentHookNameInDev = 'useEffectEvent';
        updateHookTypesDev();
        return updateEvent(callback);
      };
  }
  InvalidNestedHooksDispatcherOnMountInDEV = {
    readContext<T>(context: ReactContext<T>): T {
      warnInvalidContextAccess();
      return readContext(context);
    },
    useCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useCallback';
      warnInvalidHookAccess();
      mountHookTypesDev();
      return mountCallback(callback, deps);
    },