feat: Modern copilot UX

[OEvgeny]

Fixes #

Changelog Entry

• (Experimental) Reworked the copilot variant to align with the modern Copilot UX, in PR #5630, by @OEvgeny
  • The legacy design is temporarily available as copilot-deprecated for migration

Description

This is our first step for the modern copilot Web Chat UX. The PR re-introduces the copilot variant and makes old UX available under copilot-deprecated variant temporarily.

This covers an initial portion of changes. More to come in subsequent PRs.

Specific Changes

• renamed old tests to *.copilot-deprecated.*
• reintroduced copilot tests with the new snapshots
• renamed all occurrences of copilot to copilot-deprecated
• updated tests infra to handle both variants
• changed styles according to new figma designs

-

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[Copilot]

Pull Request Overview

This PR introduces a new copilot-deprecated variant to preserve the legacy Copilot styling while updating the copilot variant with new design specifications. The implementation adds support for both variants across the codebase, including schema updates, CSS styling, and comprehensive test coverage.

Key Changes:

• Added copilot-deprecated variant to the variant schema alongside existing copilot variant
• Split CSS styles to maintain legacy copilot-deprecated styles while introducing updated copilot styles
• Updated test configurations to use copilot-deprecated as the default comparison variant

Reviewed Changes

Copilot reviewed 37 out of 169 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/fluent-theme/src/private/VariantComposer.ts	Added copilot-deprecated to the variant schema
packages/test/page-object/src/globals/renderWebChat/index.js	Added logic to detect and handle both copilot variants
packages/fluent-theme/src/components/theme/Theme.module.css	Split styles for copilot and copilot-deprecated variants with distinct visual specifications
packages/fluent-theme/src/components/sendBox/SendBox.module.css	Applied shared text counter visibility styles to both variants
packages/fluent-theme/src/components/activity/PartGroupingDecorator.tsx	Updated header rendering logic to use copilot-deprecated
packages/fluent-theme/src/components/activity/PartGroupingDecorator.module.css	Restructured styles with shared, deprecated-specific, and new copilot-specific sections
packages/fluent-theme/src/components/activity/ActivityLoader.module.css	Applied shared loader margin styles to both variants
__tests__/html2/side-by-side/index.html	Changed default comparison variant from copilot to copilot-deprecated
Multiple test HTML files	Added new test files for copilot-deprecated variant scenarios
Multiple snapshot PNG files	Added visual regression test snapshots for the updated copilot variant

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