perf(SelectPanel): Optimize performance by improving the lookup algorithms

[hectahertz]

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)

[changeset-bot]

🦋 Changeset detected

Latest commit: 4ed9d7b

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

This PR includes changesets to release 1 package

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@primer/react	Patch

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.

[Copilot]

Pull request overview

This PR focuses on improving the performance of SelectPanel by replacing repeated linear scans over the items/selected arrays with precomputed Set-based lookups.

Changes:

• Added itemsInViewSet to speed up “Select all” handling, especially when filtering, by doing O(1) membership checks instead of items.some(...).
• Added selectedItemsSet to make isItemCurrentlySelected O(1) for multi-select panels.
• Updated itemsToRender sorting to precompute a selectedOnSortSet and use it for determining whether items should be ordered first when shouldOrderSelectedFirst is enabled.

[liuliu-dev]

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