Remove support for sx and Box usage from the UnderlineNav component

[liuliu-dev]

Closes #5770

• Note: Multiple repos are still using sx props with UnderlineNav. Tracking here: https://github.com/github/primer/issues/5827

Changelog

Removed

Removed sx props and Box from UnderlineNav.

Rollout strategy

• Major release; if selected, include a written rollout or migration plan

[changeset-bot]

🦋 Changeset detected

Latest commit: d59c187

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Major
@primer/styled-react	Major

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	89.8 KB (-0.06% 🔽)
packages/react/dist/browser.umd.js	90.05 KB (+0.07% 🔺)

[Copilot]

Pull Request Overview

This PR removes the deprecated sx prop and Box component usage from the UnderlineNav component as part of a major release. The changes replace styled-system functionality with CSS modules and native HTML elements for better performance and maintainability.

Key Changes

• Removed sx prop support from both UnderlineNav and UnderlineNavItem components
• Replaced Box components with native HTML elements (li, div, span)
• Migrated inline styles to CSS modules for better maintainability

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/UnderlineNav/styles.ts	Changed marginRight from theme unit to explicit pixel value
packages/react/src/UnderlineNav/UnderlineNavItem.tsx	Removed sx prop, Box import, replaced Box with li element and CSS module
packages/react/src/UnderlineNav/UnderlineNavItem.module.css	Added CSS module for UnderlineNavItem styling
packages/react/src/UnderlineNav/UnderlineNav.tsx	Removed sx prop, Box imports, replaced Box elements with native HTML
packages/react/src/UnderlineNav/UnderlineNav.module.css	Added CSS module for menu item content styling
packages/react/src/UnderlineNav/UnderlineNav.docs.json	Removed sx prop from documentation
.changeset/hot-mirrors-feel.md	Added changeset for major version bump

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!