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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

expect(ReactNoop.flush()).toEqual([
      'Suspend! [goodbye]',
      'Loading...',
      'Commit: goodbye',
    ]);
    expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
    ReactNoop.advanceTime(20000);
    await advanceTimers(20000);
    expect(ReactNoop.clearYields()).toEqual(['Promise resolved [goodbye]']);
    expect(ReactNoop.getChildren()).toEqual([span('Loading...')]);
    expect(ReactNoop.flush()).toEqual(['goodbye']);
    expect(ReactNoop.getChildren()).toEqual([span('goodbye')]);
  });
  describe('a Delay component', () => {
    function Never() {
      
      
      
      throw new Promise(resolve => setTimeout(() => resolve(), 10000));
    }
    function Delay({ms}) {
      
      
      return (
        <Suspense fallback={null} maxDuration={ms}>
          <Never />
        </Suspense>
      );
    }
    function DebouncedText({text, ms}) {