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`.
await waitForPaint(['shouldHideInParent: true', '(empty)']);
        expect(root).toMatchRenderedOutput('A(empty)BC');
        
        
        
        await waitForAll([
          'A',
          'shouldHideInParent: true',
          '(empty)',
          'B',
          'C',
        ]);
        expect(root).toMatchRenderedOutput('A(empty)BC');
      });
    },
  );
  
  test(
    'interrupt a refresh transition when something suspends and a parent ' +
      'component received an interleaved update after its queue was processed',
    async () => {
      
      
      
      
      function App({shouldSuspend, step}) {