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`.
getSnapshot,
      setSnapshot: (null: any),
    },
    source,
    subscribe,
  }: MutableSourceMemoizedState<Source, Snapshot>);
  return useMutableSource(hook, source, getSnapshot, subscribe);
}
function updateMutableSource<Source, Snapshot>(
  source: MutableSource<Source>,
  getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
  subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
): Snapshot {
  if (!enableUseMutableSource) {
    return (undefined: any);
  }
  const hook = updateWorkInProgressHook();
  return useMutableSource(hook, source, getSnapshot, subscribe);
}
function mountSyncExternalStore<T>(
  subscribe: (() => void) => () => void,
  getSnapshot: () => T,
  getServerSnapshot?: () => T,
): T {
  const fiber = currentlyRenderingFiber;
  const hook = mountWorkInProgressHook();
  let nextSnapshot;
  const isHydrating = getIsHydrating();
  if (isHydrating) {
    if (getServerSnapshot === undefined) {