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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

if (isHidden) {
            instance = createHiddenTextInstance(
              text,
              rootContainerInstance,
              currentHostContext,
              workInProgress,
            );
          } else {
            instance = createTextInstance(
              text,
              rootContainerInstance,
              currentHostContext,
              workInProgress,
            );
          }
          node.stateNode = instance;
        }
        appendInitialChild(parent, instance);
      } else if (node.tag === HostPortal) {
        
        
        
      } else if (node.tag === SuspenseComponent) {
        const current = node.alternate;
        if (current !== null) {
          const oldState: SuspenseState = current.memoizedState;
          const newState: SuspenseState = node.memoizedState;
          const oldIsHidden = oldState !== null && oldState.didTimeout;
          const newIsHidden = newState !== null && newState.didTimeout;
          if (oldIsHidden !== newIsHidden) {