<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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 (__DEV__) {
    ReactStrictModeWarnings.flushPendingUnsafeLifecycleWarnings();
    ReactStrictModeWarnings.flushLegacyContextWarning();
    if (warnAboutDeprecatedLifecycles) {
      ReactStrictModeWarnings.flushPendingDeprecationWarnings();
    }
  }
  while (nextEffect !== null) {
    const effectTag = nextEffect.effectTag;
    if (effectTag & (Update | Callback)) {
      recordEffect();
      const current = nextEffect.alternate;
      commitLifeCycles(
        finishedRoot,
        current,
        nextEffect,
        committedExpirationTime,
      );
    }
    if (effectTag & Ref) {
      recordEffect();
      commitAttachRef(nextEffect);
    }
    if (enableHooks && effectTag & Passive) {
      rootWithPendingPassiveEffects = finishedRoot;
    }
    nextEffect = nextEffect.nextEffect;
  }
}
function commitPassiveEffects(root: FiberRoot, firstEffect: Fiber): void {