Remove Box usage and sx prop from UnderlineNav

[mperrotti]

Relates to https://github.com/github/primer/issues/5424 and https://github.com/github/primer/issues/5425

Changelog

New

Changed

sx prop was removed from UnderlineNav and its subcomponents

Removed

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: d41cab6

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

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

[Copilot]

Pull Request Overview

This PR removes the sx prop from UnderlineNav and its subcomponents as part of the migration away from styled-components. The changes introduce a CSS Modules-based styling approach while maintaining the component's functionality.

Key Changes:

• Removes sx prop support from UnderlineNav and UnderlineNavItem components
• Introduces CSS Modules for styling with UnderlineNav.module.css
• Adds a modern polymorphic utility for better type inference with the as prop
• Creates styled-react compatibility layer to maintain backward compatibility

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/react/src/UnderlineNav/UnderlineNav.tsx	Removes Box usage and sx prop, replaces with CSS Modules classes
packages/react/src/UnderlineNav/UnderlineNavItem.tsx	Migrates from Box/sx to modern polymorphic approach with CSS classes
packages/react/src/UnderlineNav/UnderlineNav.module.css	New CSS Modules file defining component styles
packages/react/src/utils/modern-polymorphic.ts	New utility for improved polymorphic component types
packages/styled-react/src/components/UnderlineNav.tsx	Compatibility wrapper to maintain sx prop support in legacy package
packages/react/src/UnderlineNav/UnderlineNav.test.tsx	Updates tests to use new function names and constants
packages/react/src/UnderlineNav/UnderlineNav.docs.json	Removes sx prop from documentation

[Copilot]

The type casting as={as as React.ElementType} suggests a type system mismatch. Consider aligning the type definitions between UnderlineNavItem and UnderlineItem to eliminate the need for this workaround.

[mperrotti]

Tried it, couldn't figure it out

[Copilot]

The TODO comment indicates this custom shadow should be replaced with a Primer Primitives variable. Consider creating a follow-up issue to address this technical debt.

Suggested change

	box-shadow:
	/* TODO: replace custom shadow with Primer Primitives variable */
	/* stylelint-disable-next-line primer/box-shadow */
	rgba(0, 0, 0, 0.12) 0 1px 3px,
	rgba(0, 0, 0, 0.24) 0 1px 2px;
	box-shadow: var(--shadow-overlay);

[mperrotti]

No, this would be the wrong style

[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!

[hectahertz]

packages/react/src/utils/modern-polymorphic.ts‎ had me second guessing it, as we're adding yet another abstraction (so more complexity in a way) but it's simple enough that it seems useful.

Nice!