refactor(Stack): Remove sx prop support from the Stack component

[jonrohan]

Closes https://github.com/github/primer/issues/5716

Changelog

New

Changed

Removed

Remove sx prop support from the Stack component

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: 4ff2096

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Major
@primer/styled-react	Major

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]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	90.27 KB (-0.01% 🔽)
packages/react/dist/browser.umd.js	90.45 KB (+0.01% 🔺)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/396686

[primer-integration]

🟢 golden-jobs completed with status success.

[Copilot]

Pull Request Overview

This PR removes sx prop support from the Stack component by replacing the BoxWithFallback wrapper with direct element rendering. This is a breaking change that simplifies the Stack component's implementation by eliminating styled-system functionality.

Key changes:

• Replace BoxWithFallback with direct component rendering for both Stack and StackItem
• Remove the BoxWithFallback import that provided sx prop support
• Default the as prop to 'div' directly in the destructuring assignment

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/react/src/Stack/Stack.tsx	Remove BoxWithFallback wrapper and sx prop support, use direct component rendering
.changeset/young-dots-heal.md	Add major version changeset documenting the breaking change

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[Copilot]

The type assertion for StackItem should reference StackItemProps<ElementType> instead of StackProps<ElementType> to match the correct props interface.

Suggested change

	}) as PolymorphicForwardRefComponent<ElementType, StackProps<ElementType>>
	}) as PolymorphicForwardRefComponent<ElementType, StackItemProps<ElementType>>

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!