<!--
  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

onWorkCanceled = jest.fn();
      onWorkScheduled = jest.fn(() => {
        if (throwInOnWorkScheduled) {
          throw Error('Expected error onWorkScheduled');
        }
      });
      onWorkStarted = jest.fn(() => {
        if (throwInOnWorkStarted) {
          throw Error('Expected error onWorkStarted');
        }
      });
      onWorkStopped = jest.fn(() => {
        if (throwInOnWorkStopped) {
          throw Error('Expected error onWorkStopped');
        }
      });
      
      SchedulerTracing.unstable_subscribe({
        onInteractionScheduledWorkCompleted,
        onInteractionTraced,
        onWorkCanceled,
        onWorkScheduled,
        onWorkStarted,
        onWorkStopped,
      });
    }
    beforeEach(() => loadModulesForTracing());
    describe('error handling', () => {
      it('should cover errors thrown in onWorkScheduled', () => {
        function Component({children}) {