<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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 fallback={<Text text="Loading..." />}>
            <AsyncTextWithEffect text={text} ms={100} />
          </Suspense>
        );
      }
      ReactTestRenderer.create(<App text="A" />);
      expect(ReactTestRenderer).toHaveYielded(['Suspend! [A]', 'Loading...']);
      jest.advanceTimersByTime(500);
      expect(ReactTestRenderer).toHaveYielded([
        'Promise resolved [A]',
        'A',
        'Did commit: A',
      ]);
    });
    it('retries when an update is scheduled on a timed out tree', () => {
      let instance;
      class Stateful extends React.Component {
        state = {step: 1};
        render() {
          instance = this;
          return <AsyncText ms={1000} text={`Step: ${this.state.step}`} />;
        }
      }
      function App(props) {
        return (
          <Suspense maxDuration={10} fallback={<Text text="Loading..." />}>
            <Stateful />
          </Suspense>
        );