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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 (firstScheduledRoot !== null) {
        recomputeCurrentRendererTime();
        let root: FiberRoot = firstScheduledRoot;
        do {
          didExpireAtExpirationTime(root, currentRendererTime);
          // 
          root = (root.nextScheduledRoot: any);
        } while (root !== firstScheduledRoot);
      }
    }
    performWork(NoWork, true);
  } finally {
    didYield = false;
  }
}
function performSyncWork() {
  performWork(Sync, false);
}
function performWork(minExpirationTime: ExpirationTime, isYieldy: boolean) {
  // 
  // 
  findHighestPriorityRoot();
  if (isYieldy) {
    recomputeCurrentRendererTime();
    currentSchedulerTime = currentRendererTime;
    if (enableUserTimingAPI) {