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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

const primaryChildParent = newIsHidden ? node.child : node;
            if (primaryChildParent !== null) {
              appendAllChildren(parent, primaryChildParent, true, newIsHidden);
            }
            
            break branches;
          }
        }
        if (node.child !== null) {
          
          node.child.return = node;
          node = node.child;
          continue;
        }
      } else if (node.child !== null) {
        node.child.return = node;
        node = node.child;
        continue;
      }
      
      node = (node: Fiber);
      if (node === workInProgress) {
        return;
      }
      while (node.sibling === null) {
        if (node.return === null || node.return === workInProgress) {
          return;