Introduce Unified Diff display mode to Compare editor

[tobiasmelcher]

Add Unified Diff Display in Text Editor as Alternative to Classic 2-Way Compare

Implements eclipse-platform/eclipse.platform.ui#3771

Summary

This contribution introduces a new Unified Diff viewing mode for org.eclipse.compare that displays differences in a single editor pane, similar to git diff or GitHub's pull request view. When enabled, it replaces the traditional side-by-side 2-way compare editor for read-only comparisons, providing a more compact and familiar diff experience.

This is a draft implementation to gather early feedback from the Eclipse community on:

• API design and public interface shape
• Integration approach with existing compare infrastructure
• User experience and workflow expectations
• Performance and edge case handling

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What's Included

1. New Public API (org.eclipse.compare.unifieddiff)

Added a new public package with the UnifiedDiff class as the main entry point for programmatically opening unified diffs in text editors.

Supported Modes:

• REPLACE_MODE: Diffs are directly applied to the editor content. Users can keep or undo individual changes.
• OVERLAY_MODE: Source content remains unmodified. Diffs are shown via code mining annotations, and users can selectively apply or cancel individual changes.
• OVERLAY_READ_ONLY_MODE: Source content remains unmodified and read-only. Diffs are shown via code mining annotations for viewing only.

API Example:

UnifiedDiff.create(editor, sourceContent, UnifiedDiffMode.OVERLAY_READ_ONLY_MODE)
    .ignoreWhiteSpace(true)
    .open();

2. Preference Page Integration

Added a new checkbox in Preferences > General > Compare/Patch:

• "Use Unified Diff instead of 2-way compare when possible"

When enabled, eligible compare operations automatically use the unified diff viewer instead of the traditional side-by-side editor.

3. Automatic Compare Editor Integration

Modified CompareUIPlugin#openCompareEditor() to detect when unified diff should be used:

• Only applies to 2-way comparisons (left vs. right, no ancestor)
• Only applies to read-only comparisons (no editing capabilities needed)
• Automatically extracts content and opens in unified diff mode when preference is enabled

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How to Test

Prerequisites

• Eclipse workspace with a Java project tracked in a Git repository

Steps

1. Open Window > Preferences > General > Compare/Patch
2. Enable the checkbox: "Use Unified Diff instead of 2-way compare when possible"
3. In the Package Explorer, right-click a Java file
4. Select Team > Show in History
5. In the History view, select two different commits
6. Right-click and choose Compare with Each Other

Expected Result:

• Instead of the traditional side-by-side compare editor, a single editor opens showing the unified diff
• Added lines appear with green background highlighting
• Deleted lines appear as code mining annotations with red background
• Character-level changes within modified lines are highlighted with darker shades

Next Steps

• Remove reflection calls - Enhance the code mining APIs so that the current reflection calls can be removed
• Code cleanup - Polish implementation
• Bug fixes - Address issues reported by the Eclipse community
• UI/UX improvements - Enhance user experience based on the community feedback

[github-actions]

Test Results

0 files   -     54  0 suites   - 54   0s ⏱️ - 34m 49s
0 tests  -  4 546  0 ✅  -  4 523  0 💤  -  23  0 ❌ ±0 
0 runs   - 12 234  0 ✅  - 12 075  0 💤  - 159  0 ❌ ±0 

Results for commit 2f247d4. ± Comparison against base commit fce9709.

[tobiasmelcher]

[vogella]

Looks amazing