[FEATURE] Implement code diff/merge view when multiple users edit the same line simultaneously in a room

[Abhushan187]

Problem

When two users edit the same line or nearby lines in a collaborative room, the last-write-wins behavior in Firestore causes silent data loss. The user whose change arrives second overwrites the first user's work without any warning or merge opportunity.

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Proposed Improvement

Add an inline diff/merge conflict resolution UI that detects when concurrent edits collide and presents both versions side-by-side for manual resolution. The feature should:

• Detect line-level edit conflicts using operational transform or last-modified timestamps
• Display a split-pane diff view inline within the editor when conflicts are detected
• Show both versions with author labels and timestamps
• Provide "Accept Mine", "Accept Theirs", and "Merge Both" actions
• Store conflict metadata temporarily until resolved
• Fall back to last-write-wins if user ignores the conflict for >60 seconds

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Expected Impact

• Better user experience by preventing silent data loss
• Improved collaboration trust — users know their edits won't disappear
• Higher usability for competitive programming and hackathon scenarios
• Better long-term scalability with robust conflict resolution

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Possible Implementation

• Add ConflictDetector utility in src/utils/ comparing local vs remote document versions using diff-match-patch
• Extend useRoom.js to track lastModifiedBy and lastModifiedAt per line range
• Create MergeConflictPanel component with split Monaco instances showing both versions
• Add conflict state to Firestore rooms/{roomId}/conflicts/{conflictId}
• Integrate with existing useEditor hook to pause auto-sync during active conflict
• Write unit tests for conflict detection edge cases (same line, adjacent lines, multi-line blocks)

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I'm GSSoC'26 contributor, Please assign this task to me!

[aayushprsingh]

Hi, I am a GSSoC 2026 contributor and would like to work on this. this project is my primary stack. Let me know if it's available!

Here's how I'm thinking of approaching this:
I'll implement a version-check in the Firestore transaction logic to catch stale writes before they overwrite line-level data. Once a collision is detected, I'll trigger an inline split-pane using a library like diff-match-patch to highlight the delta and provide UI hooks for the user to commit the final resolution back to the shared state.

[Abhushan187]

/assign

[github-actions]

Hi @Abhushan187, this issue has been successfully assigned to you! 🎉

Make sure to read our Contributing Guidelines and submit your PR within the GSSoC timeframe. Happy coding! 🚀