<!--
  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

workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  switch (workInProgress.tag) {
    case ClassComponent: {
      const Component = workInProgress.type;
      if (isLegacyContextProvider(Component)) {
        popLegacyContext(workInProgress);
      }
      const effectTag = workInProgress.effectTag;
      if (effectTag & ShouldCapture) {
        workInProgress.effectTag = (effectTag & ~ShouldCapture) | DidCapture;
        return workInProgress;
      }
      return null;
    }
    case HostRoot: {
      popHostContainer(workInProgress);
      popTopLevelLegacyContextObject(workInProgress);
      const effectTag = workInProgress.effectTag;
      invariant(
        (effectTag & DidCapture) === NoEffect,
        'The root failed to unmount after an error. This is likely a bug in ' +