fix(SelectPanel): auto scroll when list overflows

[dgreif]

Currently, SelectPanel is allowing the list contents to overflow. This change brings back overflow: auto behavior. It also adds scrollIntoView that will trigger when active descendant changes, so that selection is always in view. I also added an example to the docs so that behavior can be tested in Vercel.

Closes https://github.com/github/primer/issues/169

Screenshots

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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🦋 Changeset detected

Latest commit: 14e04c8

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

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@primer/components	Patch

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[VanAnderson]

Pulled it down to check it out, looks good to me 👍

[smockle]

From @dgreif in #1247 (comment):

It also adds scrollIntoView that will trigger when active descendant changes, so that selection is always in view.

I’d initially misinterpreted “selection” in “so that selection is always in view” as referring to the selected item (rather than referring to the active descendent), and was surprised to discover single selection SelectPanel Items are not brought into view (either by moving them to the top of the list, or by scrolling to their list position) (screen recording):

I think this is non-blocking/out-of-scope. If this affects single selection SelectPanels outside of this one Storybook, is it something we’d want to track separately?

[dgreif]

@smockle great discovery! My initial implementation of scroll into view is focused on active-descendant, but we could definitely expand it to include scrolling to the first selected option in the list. I do think that's outside the scope of this change, but definitely worth discussing as another enhancement.