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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

root.finishedWork = null;
  // 
  // 
  if (root === lastCommittedRootDuringThisBatch) {
    // 
    // 
    nestedUpdateCount++;
  } else {
    // 
    lastCommittedRootDuringThisBatch = root;
    nestedUpdateCount = 0;
  }
  commitRoot(root, finishedWork);
}
function onUncaughtError(error: mixed) {
  invariant(
    nextFlushedRoot !== null,
    'Should be working on a root. This error is likely caused by a bug in ' +
      'React. Please file an issue.',
  );
  // 
  // 
  nextFlushedRoot.expirationTime = NoWork;
  if (!hasUnhandledError) {
    hasUnhandledError = true;
    unhandledError = error;
  }
}
// TODO: Batching should be implemented at the renderer level, not inside
// the reconciler.