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 {
  HasEffect as HookHasEffect,
  Layout as HookLayout,
  Passive as HookPassive,
  Insertion as HookInsertion,
} from './ReactHookEffectTags';
import {
  getWorkInProgressRoot,
  getWorkInProgressRootRenderLanes,
  scheduleUpdateOnFiber,
  requestUpdateLane,
  requestEventTime,
  markSkippedUpdateLanes,
  isInvalidExecutionContextForEventFunction,
} from './ReactFiberWorkLoop';
import getComponentNameFromFiber from 'react-reconciler/src/getComponentNameFromFiber';
import is from 'shared/objectIs';
import isArray from 'shared/isArray';
import {
  markWorkInProgressReceivedUpdate,
  checkIfWorkInProgressReceivedUpdate,
} from './ReactFiberBeginWork';
import {getIsHydrating} from './ReactFiberHydrationContext';
import {
  getWorkInProgressVersion,
  markSourceAsDirty,
  setWorkInProgressVersion,
  warnAboutMultipleRenderersDEV,
} from './ReactMutableSource';
import {logStateUpdateScheduled} from './DebugTracing';