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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

// 
        completeRoot(root, finishedWork, expirationTime);
      }
    }
  } else {
    // 
    let finishedWork = root.finishedWork;
    if (finishedWork !== null) {
      // 
      completeRoot(root, finishedWork, expirationTime);
    } else {
      root.finishedWork = null;
      // 
      // 
      const timeoutHandle = root.timeoutHandle;
      if (timeoutHandle !== noTimeout) {
        root.timeoutHandle = noTimeout;
        // 
        cancelTimeout(timeoutHandle);
      }
      renderRoot(root, isYieldy);
      finishedWork = root.finishedWork;
      if (finishedWork !== null) {
        // 
        // 
        if (!shouldYieldToRenderer()) {
          // 
          completeRoot(root, finishedWork, expirationTime);
        } else {
          //