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`.
}
  }
  markUpdateInDevTools(fiber, lane, action);
}
function dispatchSetState<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
): void {
  if (__DEV__) {
    if (typeof arguments[3] === 'function') {
      console.error(
        "State updates from the useState() and useReducer() Hooks don't support the " +
          'second callback argument. To execute a side effect after ' +
          'rendering, declare it in the component body with useEffect().',
      );
    }
  }
  const lane = requestUpdateLane(fiber);
  const update: Update<S, A> = {
    lane,
    action,
    hasEagerState: false,
    eagerState: null,
    next: (null: any),
  };
  if (isRenderPhaseUpdate(fiber)) {
    enqueueRenderPhaseUpdate(queue, update);
  } else {
    const alternate = fiber.alternate;