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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 <block>{this.props.label}</block>;
              }
            }
            const renderer = ReactTestRenderer.create(
              <React.unstable_Profiler id="outer" onRender={jest.fn()}>
                <FunctionComponent label="outer function component" />
                <React.unstable_Profiler id="inner" onRender={jest.fn()}>
                  <ClassComponent label="inner class component" />
                  <span>inner span</span>
                </React.unstable_Profiler>
              </React.unstable_Profiler>,
            );
            expect(renderer.toJSON()).toMatchSnapshot();
          });
        });
      });
    });
  });
  [true, false].forEach(enableSchedulerTracing => {
    describe('onRender callback', () => {
      beforeEach(() => {
        jest.resetModules();