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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 (callbackID !== null) {
        // 
        // 
        cancelDeferredCallback(callbackID);
      }
    }
    // 
  } else {
    startRequestCallbackTimer();
  }
  callbackExpirationTime = expirationTime;
  const currentMs = now() - originalStartTimeMs;
  const expirationTimeMs = expirationTimeToMs(expirationTime);
  const timeout = expirationTimeMs - currentMs;
  callbackID = scheduleDeferredCallback(performAsyncWork, {timeout});
}
// For every call to renderRoot, one of onFatal, onComplete, onSuspend, and
// onYield is called upon exiting. We use these in lieu of returning a tuple.
// I've also chosen not to inline them into renderRoot because these will
// eventually be lifted into the renderer.
function onFatal(root) {
  root.finishedWork = null;
}
function onComplete(
  root: FiberRoot,
  finishedWork: Fiber,
  expirationTime: ExpirationTime,
) {
  root.pendingCommitExpirationTime = expirationTime;
  root.finishedWork = finishedWork;