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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 (!isBatchingUpdates && !isRendering) {
      performSyncWork();
    }
  }
}
export {
  requestCurrentTime,
  computeExpirationForFiber,
  captureCommitPhaseError,
  onUncaughtError,
  renderDidSuspend,
  renderDidError,
  retrySuspendedRoot,
  markLegacyErrorBoundaryAsFailed,
  isAlreadyFailedLegacyErrorBoundary,
  scheduleWork,
  requestWork,
  flushRoot,
  batchedUpdates,
  unbatchedUpdates,
  flushSync,
  flushControlled,
  deferredUpdates,
  syncUpdates,
  interactiveUpdates,
  flushInteractiveUpdates,
  computeUniqueAsyncExpiration,
  flushPassiveEffects,
};