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`.
console.error(
        'Expected useImperativeHandle() second argument to be a function ' +
          'that creates a handle. Instead received: %s.',
        create !== null ? typeof create : 'null',
      );
    }
  }
  
  const effectDeps =
    deps !== null && deps !== undefined ? deps.concat([ref]) : null;
  let fiberFlags: Flags = UpdateEffect | LayoutStaticEffect;
  if (
    __DEV__ &&
    (currentlyRenderingFiber.mode & StrictEffectsMode) !== NoMode
  ) {
    fiberFlags |= MountLayoutDevEffect;
  }
  mountEffectImpl(
    fiberFlags,
    HookLayout,
    imperativeHandleEffect.bind(null, create, ref),
    effectDeps,
  );
}
function updateImperativeHandle<T>(
  ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
  create: () => T,
  deps: Array<mixed> | void | null,
): void {
  if (__DEV__) {