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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

const childrenUnchanged = workInProgress.firstEffect === null;
    if (childrenUnchanged) {
      
    } else {
      const container = portalOrRoot.containerInfo;
      let newChildSet = createContainerChildSet(container);
      
      appendAllChildrenToContainer(newChildSet, workInProgress, false, false);
      portalOrRoot.pendingChildren = newChildSet;
      
      markUpdate(workInProgress);
      finalizeContainerChildren(container, newChildSet);
    }
  };
  updateHostComponent = function(
    current: Fiber,
    workInProgress: Fiber,
    type: Type,
    newProps: Props,
    rootContainerInstance: Container,
  ) {
    const currentInstance = current.stateNode;
    const oldProps = current.memoizedProps;
    
    
    const childrenUnchanged = workInProgress.firstEffect === null;
    if (childrenUnchanged && oldProps === newProps) {
      
      
      workInProgress.stateNode = currentInstance;