Don't crash when container padding uses a screen that isn't a breakpoint

[kwy404]

Summary

buildCustomContainerUtilityRules (used for the legacy theme.container config and by @tailwindcss/upgrade) resolves every padding key with theme.resolveValue(key, ['--breakpoint']) and sorts the result with compareBreakpoints. A key that isn't a breakpoint resolves to null, and compareBreakpoints then calls null.indexOf('('), so the build throws TypeError: Cannot read properties of null (reading 'indexOf'). The .filter(Boolean) after the map looks like it was meant to drop those keys, but it runs on the [key, breakpoint, value] tuples, which are always truthy.

This happens with a valid v3 config where a padding key is only defined as a container screen:

container: {
  padding: { xs: '1rem', md: '2rem' },
  screens: { xs: '30rem', md: '48rem' },
}

Without custom screens, a padding key that isn't a breakpoint crashes the same way, or, when it is the only one, makes the whole container utility disappear because theme(--breakpoint-xs) can't be resolved.

The fix filters on the resolved value instead: keys that are neither a breakpoint nor a custom container screen are skipped, like in v3. With custom container screens, each padding value is added to that screen's own rule, so the sort is only done when there are no custom screens.

Test plan

• Extended two tests in packages/tailwindcss/src/compat/container-config.test.ts. Both fail before the change with the TypeError above and pass after it:
  • padding applies to custom container screens now has an xs container screen with xs padding, and the snapshot gains the @media (min-width: 30rem) rule with padding-inline: 1rem.
  • allows padding to be defined at custom breakpoints now has an xs padding key that isn't a breakpoint. The snapshot is unchanged because the key is ignored.
• vitest run src/compat/container-config.test.ts (in packages/tailwindcss)
• vitest run (in packages/tailwindcss): 42 files, 5015 tests passed
• prettier --check on the changed files

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• CHANGELOG.md

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• CHANGELOG.md

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Walkthrough

Legacy container padding keys are retained when they match a configured breakpoint or custom container screen. Unmatched keys are skipped. Custom-screen padding is added to its screen rule, and padding is sorted only when there are no custom screen overwrites. Tests cover unmatched xs padding and an xs screen at 30rem.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 95616

The reviewed container-padding behavior matches the intended handling of unknown keys and custom screens. No actionable issue remains before merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main fix: preventing crashes when container padding references a screen that is not a breakpoint.
Description check	✅ Passed	The description directly explains the crash, affected code, fix, test changes, and validation results.
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.

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: e3e0f1f9-78b6-4c01-a75e-119fdc245fde

📥 Commits

Reviewing files that changed from the base of the PR and between fa81d69 and 05eeca5.

📒 Files selected for processing (3)

• CHANGELOG.md
• packages/tailwindcss/src/compat/container-config.test.ts
• packages/tailwindcss/src/compat/container.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.

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Fixes crash in container padding configuration handling.

The PR appears safe to merge; the previously reported changelog link is corrected and no new issue was identified.

_{Reviews (2) · Last reviewed commit: "Update changelog entry with PR number"}