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`.
const newQueue: UpdateQueue<Snapshot, BasicStateAction<Snapshot>> = {
      pending: null,
      lanes: NoLanes,
      dispatch: null,
      lastRenderedReducer: basicStateReducer,
      lastRenderedState: snapshot,
    };
    newQueue.dispatch = setSnapshot = (dispatchSetState.bind(
      null,
      currentlyRenderingFiber,
      newQueue,
    ): any);
    stateHook.queue = newQueue;
    stateHook.baseQueue = null;
    snapshot = readFromUnsubscribedMutableSource(root, source, getSnapshot);
    stateHook.memoizedState = stateHook.baseState = snapshot;
  }
  return snapshot;
}
function mountMutableSource<Source, Snapshot>(
  source: MutableSource<Source>,
  getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
  subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
): Snapshot {
  if (!enableUseMutableSource) {
    return (undefined: any);
  }
  const hook = mountWorkInProgressHook();
  hook.memoizedState = ({
    refs: {