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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

nextRenderDidError = true;
}
function retrySuspendedRoot(
  root: FiberRoot,
  boundaryFiber: Fiber,
  sourceFiber: Fiber,
  suspendedTime: ExpirationTime,
) {
  let retryTime;
  if (isPriorityLevelSuspended(root, suspendedTime)) {
    // 
    retryTime = suspendedTime;
    markPingedPriorityLevel(root, retryTime);
  } else {
    // 
    const currentTime = requestCurrentTime();
    retryTime = computeExpirationForFiber(currentTime, boundaryFiber);
    markPendingPriorityLevel(root, retryTime);
  }
  // TODO: If the suspense fiber has already rendered the primary children
  // without suspending (that is, all of the promises have already resolved),
  // we should not trigger another update here. One case this happens is when
  // we are in sync mode and a single promise is thrown both on initial render
  // and on update; we attach two .then(retrySuspendedRoot) callbacks and each