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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(onInteractionTraced).toHaveBeenCalledTimes(1);
        expect(onInteractionTraced).toHaveBeenLastNotifiedOfInteraction(
          interaction,
        );
        expect(onInteractionScheduledWorkCompleted).not.toHaveBeenCalled();
        expect(getWorkForReactThreads(onWorkStarted)).toHaveLength(0);
        expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(0);
        expect(ReactNoop.flush()).toEqual([
          'Suspend [Async]',
          'Text [Loading...]',
          'Text [Sync]',
          'Monkey',
        ]);
        
        expect(ReactNoop.getChildrenAsJSX()).toEqual(null);
        expect(onRender).not.toHaveBeenCalled();
        
        
        ReactNoop.expire(10000);
        await awaitableAdvanceTimers(10000);
        
        
        expect(ReactNoop.flushExpired()).toEqual([]);
        
        expect(ReactNoop.getChildrenAsJSX()).toEqual('Loading...Sync');
        expect(onRender).toHaveBeenCalledTimes(1);
        let call = onRender.mock.calls[0];
        expect(call[0]).toEqual('test-profiler');
        expect(call[6]).toMatchInteractions(