Improve keyboard navigation in SubMenu anchor variant

[joshfarrant]

Summary

• Ensures anchor nav is visible when it contains focus
• Removes visibility: hidden from anchor nav to allow it to appear after the main SubNav items in the focus order
• Adds a storybook interaction test to assert focus order in the anchor nav variant

What should reviewers focus on?

• Test the focus order in any of the SubNav anchor variant stories
• Check that the new Storybook interaction test is passing for you

Steps to test

• Navigate to an anchor nav variant SubNav
• Navigate through the component using the keyboard (Tab)
• Note that the anchor nav receives focus after the main navigation elements, and that it becomes visible

Supporting resources (related issues, external links, etc)

• Closes https://github.com/github/primer/issues/4675

Contributor checklist

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

[changeset-bot]

🦋 Changeset detected

Latest commit: 0ff254a

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

This PR includes changesets to release 8 packages

Name	Type
@primer/react-brand	Patch
@primer/brand-docs	Patch
@primer/brand-css	Patch
@primer/brand-primitives	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	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

[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[Copilot]

Pull Request Overview

This PR improves keyboard navigation for the SubNav anchor variant by making anchor-based submenus visible when focused and adding a Storybook interaction test to verify focus order.

• Refactors the anchor variant story to use StoryFn typing and a bind-based template
• Adds a new interaction test (AnchorNavVariantKeyboardNavigation.play) that tabs through each link and verifies focus
• Updates the changeset to document the keyboard navigation enhancement

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 3 comments.

File	Description
packages/react/src/SubNav/SubNav.features.stories.tsx	Refactored story definitions, renamed template, added keyboard navigation play test
.changeset/breezy-shrimps-sell.md	Added changeset entry describing improved keyboard navigation

Files not reviewed (1)

• packages/react/src/SubNav/SubNav.module.css: Language not supported

Comments suppressed due to low confidence (1)

packages/react/src/SubNav/SubNav.features.stories.tsx:494

• Consider also asserting that the anchor navigation container is visible when focused (e.g., using .toBeVisible()) to fully validate the visibility behavior introduced in this PR.

  expect(getAllByRole('button', {name: 'Learn more'})[0]).toHaveFocus()

[rezrah]

Works great @joshfarrant 👍