[austincalvelage]

Description

Adds ESLint policies scoped to Mosaic, split into rule-specific commits. Reuses the existing TypeScript ESLint and StyleX libraries without adding dependencies, and keeps the current React 18 and React 19 contract and hooks plugin unchanged.

Rule	Level	What it checks
@typescript-eslint/consistent-type-assertions	Existing error	Mosaic already prohibits non-const assertions. This also covers assertion chains and widening followed by an assertion, so no custom rules are needed for those cases.
@typescript-eslint/no-unnecessary-condition	Warning	Conditions that cannot change the result according to their types. Limited to internal utils and machine code.
good-css/explicit-transition-properties	Error	transition: all and shorthand transitions that implicitly target every property. Standalone duration atoms remain valid for style composition.
good-css/reduced-motion	Error	Movement transitions without a recognized no-preference guard or reduced-motion override that removes the movement or sets its duration to zero.
good-css/preserve-focus-outline	Error	Explicit outline removal in focus branches without a recognized outline replacement or forced-colors restoration.
@stylexjs/valid-styles with propLimits	Error	Rejects marginTop, marginBottom, paddingTop, and paddingBottom, with messages pointing to their logical block-axis equivalents.

The new implementation checks exclude tests. The CSS checks inspect authored StyleX declarations in *.styles.ts, including conditional values, local constant media-query keys, and simple dynamic style functions. They do not inspect arbitrary objects or consumer styles.

Only the three transition, reduced-motion, and focus checks remain local. They need StyleX-specific conditional context that the installed library rules do not provide. Accumulator-copying enforcement is omitted because there is no equivalent rule in the existing dependencies.

Replaces the 11 existing physical-spacing declarations with marginBlockStart, paddingBlockStart, or paddingBlockEnd. All values stay unchanged, preserving the horizontal layout in both LTR and RTL. Existing assertion exceptions stay under Mosaic's established suppression policy.

Checklist

• Relevant tests pass: 127 ESLint tests and 43 Mosaic component tests.
• Mosaic lint and the affected styles pass; all 11 spacing warnings are resolved.
• Mosaic CSS build passes with pnpm --filter @clerk/mosaic build:css.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other: Mosaic lint policy

[changeset-bot]

🦋 Changeset detected

Latest commit: 27d8e6c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@clerk/mosaic	Patch
@clerk/swingset	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

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Project	Deployment	Actions	Updated
clerk-js-sandbox	Ready	Preview	Oct 27, 2026 5:00am UTC
swingset	Ready	Preview	Oct 27, 2026 5:00am UTC

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commit: 27d8e6c