feat: add preview of ongoing items in the group for copilot variant

[OEvgeny]

Fixes #

Changelog Entry

• (Experimental) Reworked the copilot variant to align with the modern Copilot UX, in PR #5630, by @OEvgeny, in PR #5634, by @OEvgeny, in PR #5637, by @OEvgeny,
  • The legacy design is temporarily available as copilot-deprecated for migration
  • Added loading animation for copilot, copilot-deprecated and fluent variants
  • Added preview for ongoing messages in the group for copilot variant

Description

This adds a preview of ongoing items from the activities group for the new copilot variant and shuffles a bit logics we use to display group title and status.

Specific Changes

-

• 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 adds a preview feature for ongoing (incomplete) items in activity groups for the new Copilot variant. When a group is collapsed, it displays a preview of the incomplete message with smooth CSS animations and transitions.

Key Changes

• Implements CSS animations for smooth collapsing/expanding of activity groups with preview of incomplete items
• Refactors group header logic to conditionally display incomplete message abstracts
• Adds a new CollapsibleGroupingList component to wrap activity lists with proper styling

Reviewed Changes

Copilot reviewed 10 out of 109 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
PartGroupingDecorator.module.css	Adds CSS custom properties, animations, and transitions for collapsible group preview feature with incomplete item visibility
TranscriptFocus.module.css	Adds min-height: 0 to enable proper CSS grid overflow behavior
TranscriptActivityList.tsx	Wraps component with memo for performance optimization
PartGroupingActivity.tsx	Refactors group header logic to show incomplete message abstracts and integrates new CollapsibleGroupingList
PartGroupingActivity.module.css	Sets gap to 0 for activity list (gap now managed by parent)
CollapsibleGroupingList.tsx	New component providing structured list wrapper with CSS class management
CollapsibleGrouping.module.css	Adds grid layout and margin-based spacing for list items
folded.copilot.html / folded.copilot.dark.html	New test files for copilot variant folded state
Snapshot PNG files	Binary test snapshots (not reviewable for code issues)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.