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`.
import {
  markStateUpdateScheduled,
  setIsStrictModeForDevtools,
} from './ReactFiberDevToolsHook';
import {createCache} from './ReactFiberCacheComponent';
import {
  createUpdate as createLegacyQueueUpdate,
  enqueueUpdate as enqueueLegacyQueueUpdate,
  entangleTransitions as entangleLegacyQueueTransitions,
} from './ReactFiberClassUpdateQueue';
import {
  enqueueConcurrentHookUpdate,
  enqueueConcurrentHookUpdateAndEagerlyBailout,
  enqueueConcurrentRenderForLane,
} from './ReactFiberConcurrentUpdates';
import {getTreeId} from './ReactFiberTreeContext';
import {now} from './Scheduler';
import {
  trackUsedThenable,
  checkIfUseWrappedInTryCatch,
  createThenableState,
} from './ReactFiberThenable';
import type {ThenableState} from './ReactFiberThenable';
import type {BatchConfigTransition} from './ReactFiberTracingMarkerComponent';
const {ReactCurrentDispatcher, ReactCurrentBatchConfig} = ReactSharedInternals;
export type Update<S, A> = {
  lane: Lane,
  action: A,
  hasEagerState: boolean,
  eagerState: S | null,