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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

expect(onInteractionScheduledWorkCompleted).toHaveBeenCalledTimes(2);
      expect(
        onInteractionScheduledWorkCompleted,
      ).toHaveBeenLastNotifiedOfInteraction(interactionLowPri);
      expect(getWorkForReactThreads(onWorkStarted)).toHaveLength(3);
      expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(2);
    });
    it('should trace work spawned by a commit phase lifecycle and setState callback', () => {
      let instance;
      class Example extends React.Component {
        state = {
          count: 0,
        };
        componentDidMount() {
          advanceTimeBy(10); 
          this.setState({count: 1}); 
        }
        componentDidUpdate(prevProps, prevState) {
          if (this.state.count === 2 && prevState.count === 1) {
            advanceTimeBy(10); 
            this.setState({count: 3}); 
          }
        }
        render() {
          instance = this;
          ReactTestRenderer.unstable_yield('Example:' + this.state.count);
          return null;
        }
      }
      const interactionOne = {