Add a static scroll-margin-top to a footnote item

[joshfarrant]

Summary

Added a static scroll-margin-top to Footnote.Item. This allows a small (100px) amount of space to be present between the top of the viewport and the Footnote.Item when it is navigated to.

What should reviewers focus on?

• Are you happy with the change, and the 100px value?

Steps to test:

1. Open the "return links" example in the Footnotes docs
2. Click the number [1]
3. Observe that the associated footnote is scrolled to with an offset of 100px

Supporting resources (related issues, external links, etc):

• Closes https://github.com/github/primer/issues/4669

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

Screenshots:

footnotes.mp4

[changeset-bot]

🦋 Changeset detected

Latest commit: 7e34303

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

This PR includes changesets to release 6 packages

Name	Type
@primer/react-brand	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.

[rezrah]

To your question:

Are you happy with the change, and the 100px value?

Should this be different across viewports? 100px on mobile is quite a lot of offset, should it be half of that?

[joshfarrant]

It could probably be reduced a bit, but I don't know if it necessarily needs to be responsive.

Our AnchorNav, for example, is 72px tall on all viewports and I'm not sure if we have any nav bars taller than that. We could bring the value down to 80px, but probably no lower than that.

[rezrah]

👍 sounds good, thanks

[rezrah]

🙌

[rezrah]

Forgot to mention @joshfarrant, could you please add a changeset to this PR

[joshfarrant]

Ah, great spot, thanks @rezrah

Changeset added, and margin reduced to 80px too