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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

ReactFeatureFlags.enableSchedulerTracing ? [interaction] : [],
        );
        expect(onInteractionTraced).toHaveBeenCalledTimes(1);
        expect(onInteractionScheduledWorkCompleted).not.toHaveBeenCalled();
        
        monkey.current.forceUpdate();
        expect(ReactNoop.flush()).toEqual(['Monkey']);
        expect(onRender).toHaveBeenCalledTimes(2);
        
        await awaitableAdvanceTimers(10000);
        expect(ReactNoop.flush()).toEqual([
          'Promise resolved [Async]',
          'AsyncText [Async]',
        ]);
        expect(ReactNoop.getChildrenAsJSX()).toEqual('AsyncSync');
        expect(onRender).toHaveBeenCalledTimes(3);
        call = onRender.mock.calls[2];
        expect(call[0]).toEqual('test-profiler');
        expect(call[6]).toMatchInteractions(
          ReactFeatureFlags.enableSchedulerTracing ? [interaction] : [],
        );
        expect(onInteractionTraced).toHaveBeenCalledTimes(1);
        expect(onInteractionScheduledWorkCompleted).toHaveBeenCalledTimes(1);
        expect(
          onInteractionScheduledWorkCompleted,
        ).toHaveBeenLastNotifiedOfInteraction(interaction);
      });
      it('does not prematurely complete for suspended sync renders', async () => {
        const interaction = {
          id: 0,