CRA - different test behaviour

🐛 bug report

Preflight Checklist

• I have read the
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• I agree to follow the
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  that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want
  to file, without success.

Description of the problem

I've been coding with CRA locally. After copy-pasting the code to codesandbox, I've encountered different test behaviour.

• Mocking window.fetch does not work (the call is not mocked). Mocking globalThis.fetch works, but causes an error. Locally, mocking window.fetch does work
• useEffect runs twice in the tests (probably due to strict mode?). For instance, if I have a useEffect(() => fetch(), []) the expect(fetch).toBeCalledTimes(2) will return true, while locally is not the case.

How has this issue affected you? What are you trying to accomplish?

It is frustrating that there are environment differences and that you can't control how tests are being run in codesandbox.

To Reproduce

In my provided sandbox, you can see the following observations:

1. jest.spyOn(window, 'fetch') or jest.spyOn(global, 'fetch') does not mock the fetch() call in App.jsx
2. expect(fetchMock).toBeCalledTimes(1); assertion fails

Link to sandbox: link (optional)

Your Environment

Start with the linked sandbox's src/App.jsx and src/App.test.js, focusing on the fetch mocks and the useEffect call count described in the report. Compare the sandbox test behavior with CRA's local behavior; done means fetch mocking works consistently and the effect is invoked the expected number of times.