fix: stale @tailwindcss/postcss output when input CSS changes without file mtime update

[deepshekhardas]

Cherry-pick of PR #20310

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 2c59daa6-759a-4d9e-a407-29fbae594cab

📥 Commits

Reviewing files that changed from the base of the PR and between bdcd708 and 1849cf9.

📒 Files selected for processing (3)

• CHANGELOG.md
• packages/@tailwindcss-postcss/src/index.test.ts
• packages/@tailwindcss-postcss/src/index.ts

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Walkthrough

The Tailwind PostCSS plugin now caches the last processed input CSS and triggers a full rebuild when the current CSS differs, in addition to dependency mtime checks. Tests add a shared PostCSS execution helper and cover concurrent builds and repeated processing with changed CSS input and a stable source path. The changelog documents the fix.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main fix: stale @tailwindcss/postcss output when input CSS changes without a file mtime update.
Description check	✅ Passed	The description is brief but relevant, identifying this as a cherry-pick of the related upstream PR.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

⚔️ Resolve merge conflicts

• Resolve merge conflict in branch fix/postcss-rebuild

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[greptile-apps]

Confidence Score: 5/5

Safe to merge; the change is narrowly scoped to the cache invalidation path and does not touch the compilation or output generation logic.

The fix adds a string comparison of the raw input CSS after the existing mtime loop, updating the stored value and promoting to a full rebuild only when the content has changed. The initialization value '' interacts safely with the first-build path because the mtime check independently forces rebuildStrategy = 'full' on a fresh cache entry. Error recovery (compiler = null) remains correct since the compiler is always reset before inputCss is consulted on the next call. The new regression test directly exercises the reported scenario with a stable on-disk mtime, and the helper refactoring is a straightforward cleanup with no behavioral change.

No files require special attention.

_{Reviews (1): Last reviewed commit: "rename `cachedInputCss` to `inputCss`" | Re-trigger Greptile}