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 waitFor(['A1']);
        
        startTransition(() => {
          root.render(<App shouldSuspend={false} step={2} />);
        });
        
        await waitFor([
          
          'Suspend! [Async]',
          'Loading...',
          'B1',
        ]);
        
        expect(root).toMatchRenderedOutput('A0B0C0');
        
        
        
        
        
        await waitForAll(['A2', 'B2', 'C2']);
        expect(root).toMatchRenderedOutput('A2B2C2');
      });
    },
  );
  it('should render normal pri updates scheduled after transitions before transitions', async () => {
    let updateTransitionPri;
    let updateNormalPri;
    function App() {