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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

render() {
            return this.props.children;
          }
        }
        const renderer = ReactTestRenderer.create(
          <React.unstable_Profiler id="outer" onRender={callback}>
            <Updater>
              <React.unstable_Profiler id="middle" onRender={callback}>
                <Pure>
                  <React.unstable_Profiler id="inner" onRender={callback}>
                    <div />
                  </React.unstable_Profiler>
                </Pure>
              </React.unstable_Profiler>
            </Updater>
          </React.unstable_Profiler>,
        );
        
        expect(callback).toHaveBeenCalledTimes(3);
        callback.mockReset();
        renderer.unstable_flushSync(() => {
          instance.setState({
            count: 1,
          });
        });