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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 (
        <Fragment>
          <Delay ms={ms} />
          <Text text={text} />
        </Fragment>
      );
    }
    it('works', async () => {
      ReactNoop.render(<DebouncedText text="A" ms={1000} />);
      ReactNoop.flush();
      expect(ReactNoop.getChildren()).toEqual([]);
      await advanceTimers(800);
      ReactNoop.expire(800);
      ReactNoop.flush();
      expect(ReactNoop.getChildren()).toEqual([]);
      await advanceTimers(1000);
      ReactNoop.expire(1000);
      ReactNoop.flush();
      expect(ReactNoop.getChildren()).toEqual([span('A')]);
    });
  });
  describe('sync mode', () => {
    it('times out immediately', async () => {
      function App() {