fix(cssnano-preset): disable z-index minification

[dpang314]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes CSS minification changes z-index #7548) and the maintainers have approved on my working plan.

Motivation

z-index minification can cause issues when working with the styles of third party libraries.

Test Plan

On the dogfooding page, the text would normally have its z-index minified to 3, but after these changes, it remains 100.

For comparison, one place on the Docusaurus website that already uses z-index is on the showcase page

1. Go to https://docusaurus.io/showcase and open Chrome DevTools with responsive dimensions and device set to Desktop
2. Hover over a tag (e.g. "Favorite" or "Open-Source") and then right click on it
3. If you navigate to the element in DevTools (it should have the role "tooltip")
4. On the official site the z-index will be 7, but on the site deploy of this pull request, the z-index is 500, which is what it is defined as in the CSS file.

Test links

Dogfooding: https://deploy-preview-7593--docusaurus-2.netlify.app/tests/pages/z-index-tests/
Deploy preview: https://deploy-preview-7593--docusaurus-2.netlify.app/

Related issues/PRs

#7548

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	aae5fb9
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/62a361980cbfe400084b5699
😎 Deploy Preview	https://deploy-preview-7593--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 94	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 83	🟢 100	🟢 100	🟢 100	🟢 90	Report

[Josh-Cena]

LGTM; will wait for @slorber's opinion on this. I don't think sane applications would have much bloat caused by z-index anyway (can't speak for those who add a 0 every time something doesn't work).

[slorber]

👍 thanks, looks like useful to disable this

It's marked as unsafe in CSSNano doc: https://cssnano.co/docs/optimisations/zindex/

Considering it's hard to ensure users will use be in the safe use-case, let's turn it off.

[slorber]

👍 thanks, looks like useful to disable this

It's marked as unsafe in CSSNano doc: https://cssnano.co/docs/optimisations/zindex/

Considering it's hard to ensure users will use be in the safe use-case, let's turn it off.