Fix 'Sourcemap is likely to be incorrect' warnings when using @tailwindcss/vite

[RobinMalfait]

This PR fixes an issue where a bunch of warnings would be shown related to sourcemaps.

This happens when we are dealing with CSS files that are not Tailwind CSS roots. In that case, in the transform step, we return the src of that module as-is because we didn't modify anything. However, when nothing changed, you have to return a NullValue such as undefined.

So this is a stupid little fix, but it should get rid of a bunch of annoying warnings.

Fixes: #19930

Test plan

• Added an integration test to mimic the problem
• Other tests still pass
• Tested it against the reproduction provided in @tailwindcss/vite still emits SOURCEMAP_BROKEN warnings in Nuxt/Vite builds #19930

Before:

After:

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the change is a two-line fix with a focused integration test and no impact on files that are Tailwind roots.

The fix aligns with Vite's documented transform hook contract: returning undefined skips the plugin for a given module, while returning the unchanged source string creates an unwanted sourcemap chain entry. Both affected call sites are corrected identically, and the new integration test directly reproduces the reported failure scenario using getCombinedSourcemap() to surface the warning.

No files require special attention.

_{Reviews (2): Last reviewed commit: "update changelog" | Re-trigger Greptile}

[coderabbitai]

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• CHANGELOG.md

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• CHANGELOG.md

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Walkthrough

The Tailwind Vite plugin's CSS transform handlers for both serve-mode and build-mode now return undefined when encountering non-Tailwind-root CSS files instead of returning the original source. A new test case validates that this change does not trigger sourcemap warnings when running a production build without Tailwind content roots present, even when sourcemaps are explicitly enabled.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly summarizes the main fix: preventing 'Sourcemap is likely to be incorrect' warnings in the @tailwindcss/vite plugin.
Description check	✅ Passed	The description clearly explains the issue (CSS files not Tailwind roots returning src instead of undefined), the fix, and testing approach, directly relating to the changeset.
Linked Issues check	✅ Passed	The PR fixes the exact issue from #19930 by returning undefined instead of src for non-root CSS files in the transform hook, eliminating sourcemap warnings.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing the sourcemap warning issue: test case, plugin logic fix, and changelog entry directly address #19930 requirements.

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