<!--
  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

expect(getWorkForReactThreads(onWorkStarted)[0][0]).toMatchInteractions([
        interactionOne,
      ]);
      expect(getWorkForReactThreads(onWorkStarted)[1][0]).toMatchInteractions([
        interactionOne,
      ]);
      expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(2);
      expect(getWorkForReactThreads(onWorkStopped)[0][0]).toMatchInteractions([
        interactionOne,
      ]);
      expect(getWorkForReactThreads(onWorkStopped)[1][0]).toMatchInteractions([
        interactionOne,
      ]);
      expect(onRender).toHaveBeenCalledTimes(2);
      let call = onRender.mock.calls[0];
      expect(call[0]).toEqual('test');
      expect(call[5]).toEqual(firstCommitTime);
      expect(call[6]).toMatchInteractions(
        ReactFeatureFlags.enableSchedulerTracing ? [interactionOne] : [],
      );
      call = onRender.mock.calls[1];
      expect(call[0]).toEqual('test');
      expect(call[5]).toEqual(mockNow());
      expect(call[6]).toMatchInteractions(
        ReactFeatureFlags.enableSchedulerTracing ? [interactionOne] : [],
      );
      onRender.mockClear();
      const interactionTwo = {
        id: 1,
        name: 'componentDidUpdate test',