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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

);
        }
        
        ReactNoop.renderLegacySyncRoot(<App />, () =>
          ReactNoop.yield('Did mount'),
        );
        await advanceTimers(100);
        expect(ReactNoop.clearYields()).toEqual([
          'Before',
          'Suspend! [Async: 1]',
          'After',
          'Before',
          'Sync: 1',
          'After',
          'Did mount',
          
          'Loading...',
          'Promise resolved [Async: 1]',
          'Async: 1',
        ]);
        expect(ReactNoop.getChildrenAsJSX()).toEqual(
          <React.Fragment>
            <span prop="Before" />
            <span prop="Async: 1" />
            <span prop="After" />
            <span prop="Before" />
            <span prop="Sync: 1" />
            <span prop="After" />
          </React.Fragment>,
        );