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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

return (
          <Suspense maxDuration={1000} fallback={<Text text="Loading..." />}>
            <AsyncText ms={100} text="Result" />
          </Suspense>
        );
      }
      
      ReactNoop.renderLegacySyncRoot(<App />);
      expect(ReactNoop.clearYields()).toEqual([
        'Suspend! [Result]',
        'Loading...',
      ]);
      expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
      await advanceTimers(100);
      expect(ReactNoop.expire(100)).toEqual([
        'Promise resolved [Result]',
        'Result',
      ]);
      expect(ReactNoop.getChildren()).toEqual([span('Result')]);
    });
    it('times out immediately when Suspense is in loose mode, even if the suspender is async', async () => {
      class UpdatingText extends React.Component {