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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

'Suspend! [A]',
      'Loading A...',
      'Suspend! [B]',
      'Loading B...',
    ]);
    expect(root).toMatchRenderedOutput(null);
    
    jest.advanceTimersByTime(4000);
    expect(root).toFlushWithoutYielding();
    expect(root).toMatchRenderedOutput('Loading A...Loading B...');
    
    
    
    jest.advanceTimersByTime(1000);
    
    expect(ReactTestRenderer).toHaveYielded(['Promise resolved [A]']);
    expect(root).toFlushAndYield(['A']);
    expect(root).toMatchRenderedOutput('ALoading B...');
    
    
    jest.advanceTimersByTime(1000);
    expect(ReactTestRenderer).toHaveYielded(['Promise resolved [B]']);
    expect(root).toFlushAndYield(['B']);
    expect(root).toMatchRenderedOutput('AB');
  });
  it('interrupts current render if promise resolves before current render phase', () => {
    let didResolve = false;
    let listeners = [];
    const thenable = {
      then(resolve) {