Ensure @tailwindcss/turbopack rebuilds when the input CSS changes but its mtime is unchanged

[natefzz]

Summary

Fixes #20581

The @tailwindcss/turbopack loader decides between an incremental and a full rebuild by comparing mtimes only. Turbopack can run the loader with the previous source after the input file's mtime has already changed, and then again with the current source. The first call rebuilds the compiler from the stale source and records the new mtime, so the second call sees an unchanged mtime and reuses that stale compiler. As a result, next dev keeps serving the CSS from one revision before the latest edit.

This is the same "one revision behind" symptom that was fixed for @tailwindcss/postcss in #17554, and @tailwindcss/postcss has also compared the input CSS with the previous compile since #20310. This PR adds the same inputCss check to the Turbopack loader, which was ported from the webpack loader and lacks this check.

Test plan

Added an integration test that edits the input CSS file during next dev and checks that the served CSS reflects each edit.

Without the fix, the new test fails because the served CSS still contains the previous edit:

AssertionError: expected '/* [project]/app/globals.css.css [app…' to contain 'token-two'

With the fix, both Turbopack tests pass:

pnpm build
pnpm test:integrations turbopack

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable issues were found.

_{Reviews (1) · Last reviewed commit: "Add changelog entry" · Reviewed by Greptile}

[RobinMalfait]

Thanks for the PR, and for the great investigation in the issue! You found the cause and the right fix.

We closed this PR in favor of #20582, which uses the same inputCss check. @tailwindcss/turbopack is a copy of @tailwindcss/webpack, and we want to keep the two loaders in sync. The webpack loader has the same gap (for example when sass-loader inlines a partial that you edit), so that PR fixes both loaders. It also adds turbopack to the CI matrix of the integration tests, because the tests in integrations/turbopack did not run in CI before.

_{Sent by an AI assistant on behalf of @RobinMalfait.}

[natefzz]

Thanks for the quick fix and the explanation.