[gethinwebster]

Description

Adds a secondaryContent prop to Header, as a follow-up from adding the same to ExpandableSection.

Related links, issue #, if available: n/a

How has this been tested?

• New unit tests: absent by default, rendered outside the heading element for every variant, and placed between the info link and the actions.
• Header, ExpandableSection and Container unit suites, plus the documenter, test-utils wrapper/selector snapshot suites and the functional test-utils suite, after a full build. Snapshot changes are only the new prop and finder.
• Checked the permutations page in the browser. The control is centered on the title line in every variant, within 0.5px.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.82%. Comparing base (c7e4aef) to head (26335b9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5255   +/-   ##
=======================================
  Coverage   97.82%   97.82%           
=======================================
  Files        1006     1006           
  Lines       32566    32567    +1     
  Branches    12095    12095           
=======================================
+ Hits        31857    31858    +1     
+ Misses        703      667   -36     
- Partials        6       42   +36     

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[Copilot]

🟡 Changes recommended

Passing secondaryContent={0} renders an unwrapped text node, losing the slot’s layout styles and causing findSecondaryContent() to return null despite visible content.

1 open issue

• Render zero-valued secondary content inside its slot wrapper · New

What changed in this PR

Adds a public secondaryContent slot to Header, reusing the existing layout support used by ExpandableSection.

Changes:

• Exposes secondary content beside the heading and before actions, outside the heading element.
• Adds findSecondaryContent() to Header test utilities.
• Adds unit coverage, visual permutations, and updated API snapshots.

File	Description
src/​test-utils/​dom/​header/​index.ts	Adds the secondary-content finder.
src/​header/​styles.scss	Renames slot selectors to match the public prop.
src/​header/​internal.tsx	Replaces the internal slot prop with secondaryContent.
src/​header/​interfaces.ts	Defines and documents the public prop.
src/​header/​__tests__/​header.test.tsx	Tests default absence, heading separation, and content order.
src/​expandable-section/​expandable-section-header.tsx	Passes secondary content through the new prop.
src/​__tests__/​snapshot-tests/​__snapshots__/​test-utils-selectors.test.tsx.snap	Records the new slot selector.
src/​__tests__/​snapshot-tests/​__snapshots__/​documenter.test.ts.snap	Records the prop and test-utils finder.
pages/​header/​permutations.page.tsx	Adds secondary-content visual permutations.

_{Review effort: Balanced}