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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

unhideInstance,
  unhideTextInstance,
} from './ReactFiberHostConfig';
import {
  captureCommitPhaseError,
  flushPassiveEffects,
  requestCurrentTime,
  scheduleWork,
} from './ReactFiberScheduler';
import {
  NoEffect as NoHookEffect,
  UnmountSnapshot,
  UnmountMutation,
  MountMutation,
  UnmountLayout,
  MountLayout,
  UnmountPassive,
  MountPassive,
} from './ReactHookEffectTags';
let didWarnAboutUndefinedSnapshotBeforeUpdate: Set<mixed> | null = null;
if (__DEV__) {
  didWarnAboutUndefinedSnapshotBeforeUpdate = new Set();
}
export function logError(boundary: Fiber, errorInfo: CapturedValue<mixed>) {
  const source = errorInfo.source;
  let stack = errorInfo.stack;
  if (stack === null && source !== null) {
    stack = getStackByFiberInDevAndProd(source);
  }
  const capturedError: CapturedError = {