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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

} else {
          unhideInstance(node.stateNode, node.memoizedProps);
        }
      } else if (node.tag === HostText) {
        const instance = node.stateNode;
        if (isHidden) {
          hideTextInstance(instance);
        } else {
          unhideTextInstance(instance, node.memoizedProps);
        }
      } else if (node.child !== null) {
        node.child.return = node;
        node = node.child;
        continue;
      }
      if (node === finishedWork) {
        return;
      }
      while (node.sibling === null) {
        if (node.return === null || node.return === finishedWork) {
          return;
        }
        node = node.return;
      }
      node.sibling.return = node.return;
      node = node.sibling;
    }
  }
}
function commitAttachRef(finishedWork: Fiber) {