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**Do you want to request a *feature* or report a *bug*?**
Not sure if its a bug or a feature request.

**What is the current behavior?**
The component given as a fallback to Suspense is remounted each time a lazy child resolves.

https://codesandbox.io/s/z6v6x3n1np
This example shows how a fallback component, which counts up, is mounted more than once.

**What is the expected behavior?**
The fallback is mounted once and is only unmounted when all lazy children are resolved.
In the example the counter would run up to 9.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
react@16.6.0

}
      const root = ReactTestRenderer.create(<App />, {
        unstable_isConcurrent: true,
      });
      
      expect(root).toFlushAndYield(['Suspend! [Step: 1]', 'Loading...']);
      jest.advanceTimersByTime(1000);
      expect(ReactTestRenderer).toHaveYielded(['Promise resolved [Step: 1]']);
      expect(root).toFlushAndYield(['Step: 1']);
      expect(root).toMatchRenderedOutput('Step: 1');
      
      instance.setState({step: 2});
      expect(root).toFlushAndYield(['Suspend! [Step: 2]', 'Loading...']);
      jest.advanceTimersByTime(500);
      expect(root).toMatchRenderedOutput('Loading...');
      
      instance.setState({step: 3});
      expect(root).toFlushAndYield(['Suspend! [Step: 3]']);
      expect(root).toMatchRenderedOutput('Loading...');
      jest.advanceTimersByTime(1000);
      expect(ReactTestRenderer).toHaveYielded([
        'Promise resolved [Step: 2]',
        'Promise resolved [Step: 3]',
      ]);
      expect(root).toFlushAndYield(['Step: 3']);
      expect(root).toMatchRenderedOutput('Step: 3');
    });
  });
});