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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

fiber: Fiber,
  errorInfo: CapturedValue<mixed>,
  expirationTime: ExpirationTime,
): Update<mixed> {
  const update = createUpdate(expirationTime);
  update.tag = CaptureUpdate;
  const getDerivedStateFromError = fiber.type.getDerivedStateFromError;
  if (typeof getDerivedStateFromError === 'function') {
    const error = errorInfo.value;
    update.payload = () => {
      return getDerivedStateFromError(error);
    };
  }
  const inst = fiber.stateNode;
  if (inst !== null && typeof inst.componentDidCatch === 'function') {
    update.callback = function callback() {
      if (typeof getDerivedStateFromError !== 'function') {
        
        
        
        
        
        markLegacyErrorBoundaryAsFailed(this);
      }
      const error = errorInfo.value;
      const stack = errorInfo.stack;
      logError(fiber, errorInfo);