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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

componentDidUpdate() {
        ReactNoop.yield('Commit: ' + this.props.text);
      }
      componentDidMount() {
        ReactNoop.yield('Commit: ' + this.props.text);
      }
      render() {
        return (
          <Suspense fallback={<Text text="Loading..." />}>
            <AsyncText ms={20000} text={this.props.text} />
          </Suspense>
        );
      }
    }
    ReactNoop.render(<Foo text="" />);
    ReactNoop.expire(1000);
    jest.advanceTimersByTime(1000);
    ReactNoop.render(<Foo text="go" />);
    ReactNoop.expire(1000);
    jest.advanceTimersByTime(1000);
    ReactNoop.render(<Foo text="good" />);
    ReactNoop.expire(1000);
    jest.advanceTimersByTime(1000);
    ReactNoop.render(<Foo text="goodbye" />);
    ReactNoop.advanceTime(10000);
    jest.advanceTimersByTime(10000);