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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

warnAboutUpdateOnUnmounted(fiber, false);
          break;
      }
    }
    return null;
  }
  if (enableSchedulerTracing) {
    const interactions = __interactionsRef.current;
    if (interactions.size > 0) {
      const pendingInteractionMap = root.pendingInteractionMap;
      const pendingInteractions = pendingInteractionMap.get(expirationTime);
      if (pendingInteractions != null) {
        interactions.forEach(interaction => {
          if (!pendingInteractions.has(interaction)) {
            // 
            interaction.__count++;
          }
          pendingInteractions.add(interaction);
        });
      } else {
        pendingInteractionMap.set(expirationTime, new Set(interactions));
        // 
        interactions.forEach(interaction => {
          interaction.__count++;
        });
      }
      const subscriber = __subscriberRef.current;
      if (subscriber !== null) {
        const threadID = computeThreadID(
          expirationTime,