Ensure custom --spacing-* values don't shadow spacing utility keywords

[thejamesgore]

Fixes #19722

This has been open a while and the existing fix attempt went stale — it was created via an automated agent and is flawed IMO, so gave it a crack.

If you define a custom named spacing value like --spacing-none: 0, leading-none starts outputting line-height: 0 instead of line-height: 1.

The leading utility resolves theme values against ['--leading', '--spacing'] before it ever falls back to the static none keyword, so the custom spacing value wins the lookup and the keyword is never reached.

The fix splits a spacing utility's theme keys into its own namespaces and the generic --spacing fallback, and checks staticValues between the two. Setting --leading-none explicitly still overrides everything, which keeps the documented way of customising it working. leading is the only spacing utility that uses staticValues so nothing else changes behaviour.

Worth noting #19773 is the earlier attempt. I've scoped this one to just the spacing utilities rather than changing value resolution for every functional utility based on first key position, plus that PR had a few other issues — I left a comment on the thread (#19722 (comment)) to have some discussion on the best approach.

For testing I added a new case to the leading test with --spacing-none: 0 and --spacing-big: 3rem — leading-none emits line-height: 1 and leading-big emits var(--spacing-big). It fails on main (you get line-height: var(--spacing-none)) and passes with the fix. The existing --leading-none: 2 override test is untouched and still green, and the full packages/tailwindcss suite passes locally.

[coderabbitai]

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• packages/tailwindcss/src/utilities.test.ts

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• packages/tailwindcss/src/utilities.test.ts

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Walkthrough

Functional utilities now support fallback theme namespaces for resolution and suggestions. Static values take precedence over fallback matches. Spacing utilities use namespaces before --spacing as primary keys and namespaces from --spacing as fallback keys. Regression tests verify leading-big, built-in leading-none, and explicit --leading-none precedence.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the primary fix: preventing custom spacing values from shadowing spacing utility keywords.
Description check	✅ Passed	The description directly explains the reported bug, implementation, scope, and regression tests.
Linked Issues check	✅ Passed	The changes address issue #19722 by preserving static keywords, spacing fallbacks, and explicit leading overrides.
Out of Scope Changes check	✅ Passed	The implementation, regression tests, and changelog entry are directly related to the linked issue objectives.

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[coderabbitai]

🧹 Nitpick comments (1)

packages/tailwindcss/src/utilities.test.ts (1)

25736-25739: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Cover explicit --leading-none precedence.

Add --leading-none to this theme and assert that leading-none uses it. This locks the required primary-theme precedence over the static value.

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• packages/tailwindcss/src/utilities.ts

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (3): Last reviewed commit: "Cover explicit --leading-none precedence..." | Re-trigger Greptile}