<!--
  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

jest.advanceTimersByTime(500);
        const highPriUpdateInteraction = {
          id: 1,
          name: 'hiPriUpdate',
          timestamp: mockNow(),
        };
        const originalPromise = resourcePromise;
        renderer.unstable_flushSync(() => {
          SchedulerTracing.unstable_trace(
            highPriUpdateInteraction.name,
            highPriUpdateInteraction.timestamp,
            () => {
              renderer.update(
                <React.unstable_Profiler id="app" onRender={onRender}>
                  <React.Suspense
                    maxDuration={2000}
                    fallback={<Text text="loading" />}>
                    <AsyncText text="loaded" ms={1000} />
                  </React.Suspense>
                  <Text text="updated" />
                </React.unstable_Profiler>,
              );
            },
          );
        });
        expect(ReactTestRenderer).toHaveYielded([
          'Suspend [loaded]',
          'Text [loading]',
          'Text [updated]',
        ]);