Add action to attach diff hunk to chat

[Copilot]

Adds ability to attach a specific diff hunk to chat directly from the hunk toolbar. When reviewing Copilot's code changes, users can now click the attach icon to add that exact change as context for follow-up comments.

Changes

• IModifiedFileEntryChangeHunk interface: Added getModifiedLineRange() to expose the modified line range
• DiffHunkWidget: Implements getModifiedLineRange() returning start/end line numbers from the underlying DetailedLineRangeMapping
• AddHunkToChatAction: New action in ChatEditingEditorHunk menu with:
  • Codicon.attach icon (paperclip)
  • Order 20 (renders after diff toggle at order 10)
  • Attaches hunk range to chat widget and focuses input
• Notebook hunks: Return undefined for line range (not applicable to cell-based changes)

Testing

1. Have Copilot make edits to a file
2. Hover over a diff hunk to reveal the toolbar
3. Click the paperclip icon
4. Verify the hunk's code range is attached to chat and input is focused

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ability to add Copilot-targeted comments on diffs</issue_title>
<issue_description>

It would be helpful to the ability to add a comment to Copilot directly on a diff of a code change it applied. When there are many changes, it takes quite some effort to accurately explain what my comment is referring to. Say we had an icon on the toolbar next to the diff icon, it would make it very convenient to add comments targeting that particular piece of change. Copilot would then figure out how to pass it to the underlying LLM so that it has the necessary context to apply the change.</issue_description>

<agent_instructions>This should be an action in the ChatEditingEditorHunk-menu. Use the AttachSelectionToChatAction action, give it an icon (paperclip?) let it render after the diff action</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Ability to add Copilot-targeted comments on diffs #287715

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[nidhishgajjar]

Orb Code Review (powered by GLM 5.1 on Orb Cloud)

Summary

Excellent feature implementation that adds the ability to attach specific diff hunks to chat from the hunk toolbar. Clean architecture, focused scope, and enhances the Copilot editing workflow significantly.

Architecture

✅ Interface-Driven Design: Extends IModifiedFileEntryChangeHunk with getModifiedLineRange() method
✅ Consistent Patterns: Follows existing action registration and menu integration patterns
✅ Proper Abstractions: Handles both code editor and notebook scenarios appropriately
✅ Clean Separation: Chat integration through IChatWidgetService without tight coupling

Implementation Quality

✅ Type Safety: Proper TypeScript interfaces and type checking
✅ Error Handling: Graceful handling when line range or widget unavailable
✅ Context Awareness: Appropriate preconditions using existing context keys
✅ Resource Management: No new disposables or memory concerns

// Clean interface extension
interface IModifiedFileEntryChangeHunk {
  accept(): Promise<boolean>;
  reject(): Promise<boolean>;
  getModifiedLineRange(): { startLineNumber: number; endLineNumberExclusive: number } | undefined;
}

Code Quality

✅ Focused Implementation: Each file has clear, single responsibility
✅ Consistent Naming: AddHunkToChatAction follows VS Code naming conventions
✅ Proper Imports: Only imports what's needed, no circular dependencies
✅ Icon Choice: Paperclip (Codicon.attach) is intuitive for "attach to chat"

Cross-file Impact

✅ Minimal Surface Area: Only 5 files changed, 54 additions, 0 deletions
✅ Interface Compliance: All implementers properly handle new interface requirement
✅ Backward Compatibility: No breaking changes to existing functionality
✅ Menu Integration: Clean integration into existing ChatEditingEditorHunk menu

User Experience

✅ Intuitive Workflow: Click paperclip → hunk attached to chat → input focused
✅ Visual Consistency: Uses established VS Code iconography
✅ Contextual Availability: Only appears when relevant (has modifications, not being modified)
✅ Smart Defaults: Focuses input after attachment for immediate use

Edge Cases Handled

✅ Notebook Cells: Returns undefined for line ranges (not applicable)
✅ Missing Widget: Graceful handling when chat widget unavailable
✅ No Line Range: Proper early return when hunk has no modifiable range
✅ Range Construction: Correctly constructs Range from line numbers

Testing Considerations

✅ Clear Test Plan: PR description provides step-by-step testing instructions
✅ Multiple Scenarios: Should work with both single and multi-line hunks
✅ Integration Points: Tests chat attachment and input focus behavior

Assessment

Recommendation: ✅ APPROVE

This is a textbook example of excellent feature development:

1. Clear User Value: Solves a real problem (difficult to reference specific changes in chat)
2. Clean Architecture: Proper interface extension with consistent implementation
3. Focused Scope: Does exactly what's needed, nothing more
4. Quality Implementation: Type-safe, error-handled, follows conventions
5. Good UX: Intuitive icon placement and workflow

Strengths:

• Interface design allows for different hunk types (code vs notebook)
• Proper separation of concerns between editing and chat systems
• Excellent use of existing VS Code infrastructure
• No over-engineering or unnecessary complexity

Minor Observations (non-blocking):

• Consider adding telemetry for feature usage tracking
• Documentation in interface comment is helpful

Ready to merge - this enhances the Copilot editing experience with zero risk and high value.

User Impact: Developers can now easily attach specific code changes to chat for targeted feedback, making the Copilot editing workflow much more efficient.