Remove sx prop in UnderlinePanels

[pksjce]

Closes ##6804

Changelog

New

Changed

Removed

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: 5e5e44f

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

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[Copilot]

Pull Request Overview

This PR removes the sx prop from the UnderlinePanels component to simplify its API and reduce dependencies on the styled-components system. The changes focus on eliminating SxProp types and BoxWithFallback usage while transitioning to standard HTML element props.

Key Changes:

• Removed SxProp from UnderlinePanelsProps and TabProps type definitions
• Replaced BoxProps with React.HTMLAttributes for PanelProps
• Simplified Panel component implementation by removing BoxWithFallback dependency

[Copilot]

The Panel component no longer accepts or forwards any props. This breaks the component's functionality as it should receive and forward props like className, children, and other HTML attributes to the div element.

Suggested change

	const Panel: FC<PanelProps> = () => {
	return <div role="tabpanel" />
	const Panel: FC<PanelProps> = (props) => {
	return <div role="tabpanel" {...props}>{props.children}</div>

[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	88.64 KB (0%)
packages/react/dist/browser.umd.js	88.73 KB (0%)