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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 ref = finishedWork.ref;
  if (ref !== null) {
    const instance = finishedWork.stateNode;
    let instanceToUse;
    switch (finishedWork.tag) {
      case HostComponent:
        instanceToUse = getPublicInstance(instance);
        break;
      default:
        instanceToUse = instance;
    }
    if (typeof ref === 'function') {
      ref(instanceToUse);
    } else {
      if (__DEV__) {
        if (!ref.hasOwnProperty('current')) {
          warningWithoutStack(
            false,
            'Unexpected ref object provided for %s. ' +
              'Use either a ref-setter function or React.createRef().%s',
            getComponentName(finishedWork.type),
            getStackByFiberInDevAndProd(finishedWork),
          );
        }
      }
      ref.current = instanceToUse;
    }
  }
}
function commitDetachRef(current: Fiber) {