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`.
isSubsetOfLanes,
  includesBlockingLane,
  includesOnlyNonUrgentLanes,
  claimNextTransitionLane,
  mergeLanes,
  removeLanes,
  intersectLanes,
  isTransitionLane,
  markRootEntangled,
  markRootMutableRead,
  NoTimestamp,
} from './ReactFiberLane';
import {
  ContinuousEventPriority,
  getCurrentUpdatePriority,
  setCurrentUpdatePriority,
  higherEventPriority,
} from './ReactEventPriorities';
import {readContext, checkIfContextChanged} from './ReactFiberNewContext';
import {HostRoot, CacheComponent} from './ReactWorkTags';
import {
  LayoutStatic as LayoutStaticEffect,
  Passive as PassiveEffect,
  PassiveStatic as PassiveStaticEffect,
  StaticMask as StaticMaskEffect,
  Update as UpdateEffect,
  StoreConsistency,
  MountLayoutDev as MountLayoutDevEffect,
  MountPassiveDev as MountPassiveDevEffect,
} from './ReactFiberFlags';