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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

// 
  // 
  if (isYieldy) {
    callbackExpirationTime = NoWork;
    callbackID = null;
  }
  // 
  if (nextFlushedExpirationTime !== NoWork) {
    scheduleCallbackWithExpirationTime(
      ((nextFlushedRoot: any): FiberRoot),
      nextFlushedExpirationTime,
    );
  }
  // 
  finishRendering();
}
function flushRoot(root: FiberRoot, expirationTime: ExpirationTime) {
  invariant(
    !isRendering,
    'work.commit(): Cannot commit while already rendering. This likely ' +
      'means you attempted to commit from inside a lifecycle method.',
  );
  // 
  // 
  // 
  nextFlushedRoot = root;
  nextFlushedExpirationTime = expirationTime;
  performWorkOnRoot(root, expirationTime, false);
  // 
  performSyncWork();