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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

jest.advanceTimersByTime(ms);
        
        
        return Promise.resolve().then(() => {});
      }
      it('traces both the temporary placeholder and the finished render for an interaction', async () => {
        loadModulesForTracing({useNoopRenderer: true});
        const interaction = {
          id: 0,
          name: 'initial render',
          timestamp: mockNow(),
        };
        const monkey = React.createRef();
        class Monkey extends React.Component {
          render() {
            ReactNoop.yield('Monkey');
            return null;
          }
        }
        const onRender = jest.fn();
        SchedulerTracing.unstable_trace(interaction.name, mockNow(), () => {
          ReactNoop.render(
            <React.unstable_Profiler id="test-profiler" onRender={onRender}>
              <React.Suspense fallback={<Text text="Loading..." />}>
                <AsyncText text="Async" ms={20000} />
              </React.Suspense>
              <Text text="Sync" />
              <Monkey ref={monkey} />
            </React.unstable_Profiler>,
          );