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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

loadModules({enableSchedulerTracing});
      });
      it('should handle errors thrown', () => {
        const callback = jest.fn(id => {
          if (id === 'throw') {
            throw Error('expected');
          }
        });
        let didMount = false;
        class ClassComponent extends React.Component {
          componentDidMount() {
            didMount = true;
          }
          render() {
            return this.props.children;
          }
        }
        
        
        expect(() =>
          ReactTestRenderer.create(
            <ClassComponent>
              <React.unstable_Profiler id="do-not-throw" onRender={callback}>
                <React.unstable_Profiler id="throw" onRender={callback}>
                  <div />
                </React.unstable_Profiler>
              </React.unstable_Profiler>
            </ClassComponent>,
          ),
        ).toThrow('expected');