Fix null font options in legacy theme resolution

[jiwoo1222]

Summary

Fix the null check in resolveThemeValue: typeof value[1] !== null always evaluates to true, so null is incorrectly treated as a font-options object.

If a JS config extends an existing default sans or mono font with a null second array element, resolving the default font settings throws while reading fontFeatureSettings. Check the value itself before accessing its properties.

Add regression tests for both font namespaces. This is defensive handling of malformed options, not a new config format.

Test plan

• The two regression cases fail with the original implementation and pass with this fix.
• pnpm exec vitest run --project tailwindcss packages/tailwindcss/src/compat/apply-config-to-theme.test.ts — 9 passed.
• Prettier check on both changed files — passed.
• pnpm exec vitest run --project tailwindcss — 5,143 passed, 3 failed, 1 skipped. The same three snapshot failures reproduce on unmodified upstream f7f47f8 (CSS mixins, rounded utilities, and theme string indexing), which has 5,141 passing tests.
• Package typecheck does not pass in this partial workspace install due to missing workspace dependencies and cross-project type errors. Rust tests and the full monorepo build have not been run.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: 608f7375-842a-4e17-9841-41b7f3df87b7

📥 Commits

Reviewing files that changed from the base of the PR and between f7f47f8 and 3de46c5.

📒 Files selected for processing: 2

• packages/tailwindcss/src/compat/apply-config-to-theme.test.ts
• packages/tailwindcss/src/compat/apply-config-to-theme.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 9 remain after this review.

Walkthrough

The theme resolver now checks whether a tuple’s second element is non-null. Regression tests cover sans and mono font configurations with a null option and verify that applying the configuration does not throw.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 3de46

Null font-option tuples are handled defensively, with regression tests for both font namespaces. No actionable merge-blocking risk is identified.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly identifies the main change: fixing null font options during legacy theme resolution.
Description check		The description directly explains the bug, the implementation fix, the regression tests, and the reported test results.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge.

_{Reviews (1) · Last reviewed commit: "Fix null font options in legacy theme re..." · Reviewed by Greptile}

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[RobinMalfait]

Thanks for the PR!

You are right that typeof value[1] !== null is always true. But ['Inter', null] is not a valid fontFamily value, and Tailwind CSS v3 also renders it as invalid CSS (font-family: Inter,;).

With this change, the build does not throw anymore, but the output is still wrong: --default-font-family becomes Inter, (with a trailing comma), and the font-sans utility ignores the config value. So the change replaces an error with invalid CSS, for a config that is already invalid.

Because of this, I am going to close this PR.

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