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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

}
function onSuspend(
  root: FiberRoot,
  finishedWork: Fiber,
  suspendedExpirationTime: ExpirationTime,
  rootExpirationTime: ExpirationTime,
  msUntilTimeout: number,
): void {
  root.expirationTime = rootExpirationTime;
  if (msUntilTimeout === 0 && !shouldYieldToRenderer()) {
    // 
    root.pendingCommitExpirationTime = suspendedExpirationTime;
    root.finishedWork = finishedWork;
  } else if (msUntilTimeout > 0) {
    // 
    root.timeoutHandle = scheduleTimeout(
      onTimeout.bind(null, root, finishedWork, suspendedExpirationTime),
      msUntilTimeout,
    );
  }
}
function onYield(root) {
  root.finishedWork = null;
}
function onTimeout(root, finishedWork, suspendedExpirationTime) {
  // 
  root.pendingCommitExpirationTime = suspendedExpirationTime;
  root.finishedWork = finishedWork;
  // 
  //