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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

} from './ReactFiberPendingPriority';
import {
  recordEffect,
  recordScheduleUpdate,
  startRequestCallbackTimer,
  stopRequestCallbackTimer,
  startWorkTimer,
  stopWorkTimer,
  stopFailedWorkTimer,
  startWorkLoopTimer,
  stopWorkLoopTimer,
  startCommitTimer,
  stopCommitTimer,
  startCommitSnapshotEffectsTimer,
  stopCommitSnapshotEffectsTimer,
  startCommitHostEffectsTimer,
  stopCommitHostEffectsTimer,
  startCommitLifeCyclesTimer,
  stopCommitLifeCyclesTimer,
} from './ReactDebugFiberPerf';
import {createWorkInProgress, assignFiberPropertiesInDEV} from './ReactFiber';
import {onCommitRoot} from './ReactFiberDevToolsHook';
import {
  NoWork,
  Sync,
  Never,
  msToExpirationTime,
  expirationTimeToMs,
  computeAsyncExpiration,
  computeInteractiveExpiration,