<!--
  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

while (node !== null) {
      
      branches: if (node.tag === HostComponent) {
        let instance = node.stateNode;
        if (needsVisibilityToggle) {
          const props = node.memoizedProps;
          const type = node.type;
          if (isHidden) {
            
            instance = cloneHiddenInstance(instance, type, props, node);
          } else {
            
            
            
            instance = cloneUnhiddenInstance(instance, type, props, node);
          }
          node.stateNode = instance;
        }
        appendInitialChild(parent, instance);
      } else if (node.tag === HostText) {
        let instance = node.stateNode;
        if (needsVisibilityToggle) {
          const text = node.memoizedProps;
          const rootContainerInstance = getRootHostContainer();
          const currentHostContext = getHostContext();