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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

switch (ReactCurrentFiber.phase) {
      case 'getChildContext':
        if (didWarnSetStateChildContext) {
          return;
        }
        warningWithoutStack(
          false,
          'setState(...): Cannot call setState() inside getChildContext()',
        );
        didWarnSetStateChildContext = true;
        break;
      case 'render':
        if (didWarnAboutStateTransition) {
          return;
        }
        warningWithoutStack(
          false,
          'Cannot update during an existing state transition (such as within ' +
            '`render`). Render methods should be a pure function of props and state.',
        );
        didWarnAboutStateTransition = true;
        break;
    }
  };
}
let lastUniqueAsyncExpiration: number = Sync - 1;
let expirationContext: ExpirationTime = NoWork;
let isWorking: boolean = false;
let nextUnitOfWork: Fiber | null = null;
let nextRoot: FiberRoot | null = null;