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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

case ClassComponent: {
      safelyDetachRef(current);
      const instance = current.stateNode;
      if (typeof instance.componentWillUnmount === 'function') {
        safelyCallComponentWillUnmount(current, instance);
      }
      return;
    }
    case HostComponent: {
      safelyDetachRef(current);
      return;
    }
    case HostPortal: {
      
      
      
      if (supportsMutation) {
        unmountHostComponents(current);
      } else if (supportsPersistence) {
        emptyPortalContainer(current);
      }
      return;
    }
  }
}
function commitNestedUnmounts(root: Fiber): void {