Closes https://github.com/github/primer/issues/5788

This PR improves the performance of SelectPanel by optimizing how selected items are looked up during rendering and sorting.

The previous implementation used O(n) or O(n*m) array lookups when:

• Checking if an item is currently selected (isItemCurrentlySelected)
• Sorting selected items to the top (shouldOrderSelectedFirst)
• Filtering selected items not in the current view (handleSelectAllChange)
• For panels with many items or selections, this caused noticeable performance degradation.

Changelog

First, the sorting algorithm in itemsToRender was slow. For each comparison during sorting, it was calling selectedOnSort.some with Object.entries and every, checking all properties of all selected items. I replaced this with a pre-computed Set of selected item IDs that allows instant lookups.

Second, the isItemCurrentlySelected function was iterating through all selected items every time it was called. Since it gets called for every item during the map operation, this was doing a lot of redundant work. I added a memoized selectedItemsSet that gets computed once when the selected prop changes, making each lookup instant.

Third, the handleSelectAllChange function had the same problem. It was using items.some inside a filter, iterating through all visible items for each selected item. I added a memoized itemsInViewSet to make those lookups instant as well.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)