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`.
enableDebugTracing,
  enableSchedulingProfiler,
  enableCache,
  enableUseRefAccessWarning,
  enableLazyContextPropagation,
  enableUseMutableSource,
  enableTransitionTracing,
  enableUseHook,
  enableUseMemoCacheHook,
  enableUseEffectEventHook,
  enableLegacyCache,
  debugRenderPhaseSideEffectsForStrictMode,
} from 'shared/ReactFeatureFlags';
import {
  REACT_CONTEXT_TYPE,
  REACT_SERVER_CONTEXT_TYPE,
  REACT_MEMO_CACHE_SENTINEL,
} from 'shared/ReactSymbols';
import {
  NoMode,
  ConcurrentMode,
  DebugTracingMode,
  StrictEffectsMode,
  StrictLegacyMode,
} from './ReactTypeOfMode';
import {
  NoLane,
  SyncLane,
  OffscreenLane,
  NoLanes,