feat: add leadingVisual to UnderlineNav.Item

[joshblack]

Closes https://github.com/github/primer/issues/5289

Add support for leadingVisual to UnderlineNav.Item and update existing references for icon to leadingVisual.

Changelog

New

• Add support for leadingVisual to UnderlineNav.Item with the React.ReactElement type

Changed

• Deprecate the icon prop in favor of leadingVisual

Removed

Rollout strategy

• Minor release

[changeset-bot]

🦋 Changeset detected

Latest commit: 47ebc3b

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Minor

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.

[Copilot]

Pull Request Overview

This PR adds support for the leadingVisual prop to UnderlineNav.Item while deprecating the existing icon prop, aligning the component with Primer's design system conventions used in other components.

Key Changes:

• Introduces leadingVisual prop accepting React.ReactElement as the new API for rendering visuals before item text
• Deprecates the icon prop with a clear migration path to leadingVisual
• Updates all internal usages across tests and stories from function components to JSX elements

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/UnderlineNav/UnderlineNavItem.tsx	Adds leadingVisual prop, marks icon as deprecated, implements backwards-compatible fallback logic
packages/react/src/UnderlineNav/UnderlineNav.test.tsx	Updates test data types from FC<IconProps> to React.ReactElement, adds backwards compatibility test for deprecated icon prop
packages/react/src/UnderlineNav/UnderlineNav.features.stories.tsx	Migrates story examples from icon to leadingVisual, updates data types to use React elements
packages/react/src/UnderlineNav/UnderlineNav.examples.stories.tsx	Migrates examples from icon to leadingVisual, removes unused Octicon wrapper, updates data types
packages/react/src/UnderlineNav/UnderlineNav.docs.json	Adds documentation for leadingVisual prop and marks icon as deprecated
.changeset/old-yaks-win.md	Documents the minor version change for the new feature

[siddharthkp]

Makes sense!

completely optional, i know you have other work as well: Would love to see a eslint rule to go with it!

[joshblack]

@siddharthkp good point, I made an issue for this over at: https://github.com/github/primer/issues/6144 to see if we could add support for repurposing the no-deprecated-props rule 👀

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/7243

[primer-integration]

🟢 ci completed with status success.

[github-actions]

👋 Hi, there are new commits since the last successful integration test. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.