fix: add tests to demonstrate that PostCSS configs are not reused across projects

[fwouts]

Description

This sets up two separate apps with conflicting PostCSS configs, along with tests to make sure that each gets only its own CSS applied.

Additional context

See vitejs#3906 for the original PR.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[fwouts]

@patak-js here are some tests for the PostCSS config caching PR. Let me know what you think about this approach, happy to reshuffle things around.

[y1d7ng]

@fwouts Thank you very much for the test. But I found that even if the original PR code is not included, the test can still be executed successfully in the current vite project.

[fwouts]

Hey @OneNail, thanks for catching this. I could have sworn I had run these tests without your commit and confirmed they failed!

I've been debugging this for an hour and I can't figure out what's going on. It looks like Jest is reloading modules between tests and resetting the value of cachedPostcssConfig every time, even when running with --runInBand which I've confirmed reuses the same process.

I ended up implementing it with a different approach to guarantee that it won't happen.

[y1d7ng]

@fwouts Thank you very much for the test. If the original PR code isn't included, the test failed at

expect(await getColor(blueA)).toBe('blue')

is it supposed to failed at

expect(await getColor(greenB)).toBe('green')

?

[fwouts]

@fwouts Thank you very much for the test. If the original PR code isn't included, the test failed at

expect(await getColor(blueA)).toBe('blue')

is it supposed to failed at

expect(await getColor(greenB)).toBe('green')

?

No, that's expected! That's because there's already another Vite server running (the one set up in https://github.com/vitejs/vite/blob/main/scripts/jestPerTestSetup.ts) so a null PostCSS config has already been cached. Therefore, even the first app fails.

I preferred to have both the blue and green app so we don't rely on this implicit behaviour :)

[y1d7ng]

@fwouts Thank you very much, I merged the commit.