[WIP] fix: address accessibility issue with dark theme using token background colors

[Copilot]

This pull request introduces a new PortalContext to the @primer/react library, allowing more flexible and context-driven control over where portal content is rendered. The changes include updates to the core Portal component, its exports, new Storybook examples, and additional tests to ensure correct behavior when using PortalContext and container overrides.

Changelog

New

• Added PortalContext to Portal.tsx, allowing consumers to specify a portalContainerName via React context, which determines the portal root container for content rendering. The Portal component now prioritizes the context value unless overridden by its own containerName prop.
• Added new tests in Portal.test.tsx to verify correct portal rendering behavior when using PortalContext, including context-only, default fallback, undefined context, and prop override scenarios.
• Added a Storybook example (WithPortalContext) in Portal.features.stories.tsx to demonstrate how to use PortalContext for controlling portal rendering, including context and prop override cases.

Changed

• Updated exports in index.ts and the package entry point to include PortalContext, making it available for consumers.
• Fixed accessibility issues in the WithPortalContext Storybook story by replacing hardcoded background and border colors with proper Primer design tokens, ensuring the story adapts correctly to both light and dark themes with excellent color contrast.

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

Accessibility Fix

The Portal context story previously used hardcoded colors (#f0f8ff, #e6f3ff, etc.) that didn't adapt to dark theme, causing accessibility issues. This has been resolved by:

• Replacing all inline styles with CSS classes using semantic design tokens
• Using var(--bgColor-accent-muted), var(--bgColor-attention-emphasis), etc. for backgrounds
• Applying var(--borderColor-*-emphasis) for borders
• Adding var(--fgColor-onEmphasis) for proper text contrast on emphasis backgrounds

Screenshots:

• Light Theme:
• Dark Theme:

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)

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[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	88.61 KB (0%)
packages/react/dist/browser.umd.js	88.66 KB (0%)

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	88.61 KB (0%)
packages/react/dist/browser.umd.js	88.66 KB (0%)