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`.
throw new Error(
        'Missing getServerSnapshot, which is required for ' +
          'server-rendered content. Will revert to client rendering.',
      );
    }
    nextSnapshot = getServerSnapshot();
    if (__DEV__) {
      if (!didWarnUncachedGetSnapshot) {
        if (nextSnapshot !== getServerSnapshot()) {
          console.error(
            'The result of getServerSnapshot should be cached to avoid an infinite loop',
          );
          didWarnUncachedGetSnapshot = true;
        }
      }
    }
  } else {
    nextSnapshot = getSnapshot();
    if (__DEV__) {
      if (!didWarnUncachedGetSnapshot) {
        const cachedSnapshot = getSnapshot();
        if (!is(nextSnapshot, cachedSnapshot)) {
          console.error(
            'The result of getSnapshot should be cached to avoid an infinite loop',
          );
          didWarnUncachedGetSnapshot = true;