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`.
updateEffectImpl(PassiveEffect, HookPassive, create, deps);
}
function useEffectEventImpl<Args, Return, F: (...Array<Args>) => Return>(
  payload: EventFunctionPayload<Args, Return, F>,
) {
  currentlyRenderingFiber.flags |= UpdateEffect;
  let componentUpdateQueue: null | FunctionComponentUpdateQueue =
    (currentlyRenderingFiber.updateQueue: any);
  if (componentUpdateQueue === null) {
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber.updateQueue = (componentUpdateQueue: any);
    componentUpdateQueue.events = [payload];
  } else {
    const events = componentUpdateQueue.events;
    if (events === null) {
      componentUpdateQueue.events = [payload];
    } else {
      events.push(payload);
    }
  }
}
function mountEvent<Args, Return, F: (...Array<Args>) => Return>(
  callback: F,
): F {
  const hook = mountWorkInProgressHook();
  const ref = {impl: callback};
  hook.memoizedState = ref;
  
  return function eventFn() {
    if (isInvalidExecutionContextForEventFunction()) {