Tests: Fix PostCSS browser-externalized warnings in Vitest

[ciampo]

Follow-up to #80855.

Upstream tracking:

• Browser-externalized warnings: vitejs/vite#23512.
• Separate CSS Modules missing-from warning: proposed fix madyankin/postcss-modules#173, related issue vitejs/vite#15410.

What?

Remove PostCSS's browser-externalized path, url, source-map-js, and fs warnings from successful Vitest Browser Mode runs.

Why?

PostCSS explicitly disables these modules in its browser mappings. Vite replaces them with proxies that warn when PostCSS checks which APIs are available. The LinkControl keyboard tests reproduce 23 warnings despite passing. PostCSS's public entry point produces the same warnings.

How?

Honor the installed PostCSS package's disabled browser imports during Browser dependency optimization. Resolution for other packages and the existing console enforcement stay unchanged.

The temporary resolver links to the upstream issue. Once the installed Vite version fixes it, remove postcss-browser-plugin.mjs and its optimizer registration. Keep the Browser regression test and rerun the full Browser suite to verify removal.

The regression imports the real editor style transformer inside the test, checks for warnings, and verifies selector prefixing and asset URL rebasing. Existing tests provide broader coverage.

Testing Instructions

1. Run npm run test:unit -- --project=browser test/unit/config/postcss.browser.test.js packages/block-editor/src/components/link-control/test/keyboard-selection.browser.test.js. Both files should pass without browser-externalized warnings. The new regression fails with the resolver removed.
2. Run npm run test:unit -- packages/block-editor/src/utils/test/transform-styles.js.
3. Run the full Browser suite with npm run test:unit -- --project=browser --maxWorkers=4. The worker limit retains every test and reduces local CPU contention.

The separate missing-from warning can still appear. It also reproduces on trunk and remains pending the CSS Modules fix linked above.

Import trace and verification

The LinkControl search input imports the components barrel, which exports __unstableEditorStyles. That loads transformStyles, then postcss/lib/processor. Through lazy-result, this reaches PostCSS's map-generator, input, and previous-map, which probe the disabled APIs during initialization.

The resolver reads the installed PostCSS browser mappings through the block-editor workspace. It affects only Browser dependency optimization. Production imports and Node/jsdom resolution are unchanged.

After rebasing onto trunk 72e0cc4f894, a fresh dependency install and full Browser run passed all 122 files with zero browser-externalized warnings. The 30 existing transformer tests, build, typecheck, targeted lint, test conventions, and test routing also passed.

Two pre-existing diagnostics remain: PostCSS's missing from option and Vite's unanalyzable dynamic import in use-widget-types.ts. Npm installation diagnostics, failure artifacts, and console enforcement are outside this change.

Use of AI Tools

Codex investigated, implemented, and verified the change. An independent Codex agent performed an adversarial review.

[github-actions]

🤖 PR meta 🤖

📦 Bundle size

Size Change: -16 B (0%)

Total Size: 8.22 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	613 kB	-16 B (0%)

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[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: manzoorwanijk <manzoorwanijk@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

I'm seeing the following when running the first steps. Is this expected?

A PostCSS plugin did not pass the `from` option to `postcss.parse`. This may cause imported assets to be incorrectly transformed. If you've recently added a PostCSS plugin that raised this warning, please contact the package author to fix the issue.

[ciampo]

@Mamaduka Yes, I reproduced this on trunk too. The CSS Modules warning has a proposed fix in postcss-modules#173, with the related asset-path issue tracked in vite#15410.

This PR addresses the separate browser-externalized warnings tracked in vite#23512. I've linked both problems in the description and code comments.

[manzoorwanijk]

This looks good now.