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`.
type StoreInstance<T> = {
  value: T,
  getSnapshot: () => T,
};
type StoreConsistencyCheck<T> = {
  value: T,
  getSnapshot: () => T,
};
type EventFunctionPayload<Args, Return, F: (...Array<Args>) => Return> = {
  ref: {
    eventFn: F,
    impl: F,
  },
  nextImpl: F,
};
export type FunctionComponentUpdateQueue = {
  lastEffect: Effect | null,
  events: Array<EventFunctionPayload<any, any, any>> | null,
  stores: Array<StoreConsistencyCheck<any>> | null,
  
  memoCache?: MemoCache | null,
};
type BasicStateAction<S> = (S => S) | S;
type Dispatch<A> = A => void;
let renderLanes: Lanes = NoLanes;
let currentlyRenderingFiber: Fiber = (null: any);
let currentHook: Hook | null = null;
let workInProgressHook: Hook | null = null;
let didScheduleRenderPhaseUpdate: boolean = false;
let didScheduleRenderPhaseUpdateDuringThisPass: boolean = false;