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`.
next: Update<S, A>,
};
export type UpdateQueue<S, A> = {
  pending: Update<S, A> | null,
  lanes: Lanes,
  dispatch: (A => mixed) | null,
  lastRenderedReducer: ((S, A) => S) | null,
  lastRenderedState: S | null,
};
let didWarnAboutMismatchedHooksForComponent;
let didWarnUncachedGetSnapshot: void | true;
let didWarnAboutUseWrappedInTryCatch;
if (__DEV__) {
  didWarnAboutMismatchedHooksForComponent = new Set<string | null>();
  didWarnAboutUseWrappedInTryCatch = new Set<string | null>();
}
export type Hook = {
  memoizedState: any,
  baseState: any,
  baseQueue: Update<any, any> | null,
  queue: any,
  next: Hook | null,
};
export type Effect = {
  tag: HookFlags,
  create: () => (() => void) | void,
  destroy: (() => void) | void,
  deps: Array<mixed> | void | null,
  next: Effect,
};