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`.
useMemo: throwInvalidHookError,
  useReducer: throwInvalidHookError,
  useRef: throwInvalidHookError,
  useState: throwInvalidHookError,
  useDebugValue: throwInvalidHookError,
  useDeferredValue: throwInvalidHookError,
  useTransition: throwInvalidHookError,
  useMutableSource: throwInvalidHookError,
  useSyncExternalStore: throwInvalidHookError,
  useId: throwInvalidHookError,
};
if (enableCache) {
  (ContextOnlyDispatcher: Dispatcher).useCacheRefresh = throwInvalidHookError;
}
if (enableUseHook) {
  (ContextOnlyDispatcher: Dispatcher).use = throwInvalidHookError;
}
if (enableUseMemoCacheHook) {
  (ContextOnlyDispatcher: Dispatcher).useMemoCache = throwInvalidHookError;
}
if (enableUseEffectEventHook) {
  (ContextOnlyDispatcher: Dispatcher).useEffectEvent = throwInvalidHookError;
}
const HooksDispatcherOnMount: Dispatcher = {
  readContext,
  useCallback: mountCallback,
  useContext: readContext,
  useEffect: mountEffect,
  useImperativeHandle: mountImperativeHandle,
  useLayoutEffect: mountLayoutEffect,