feat: CodeMirror 6 side-by-side diff editor

[PureWeen]

CodeMirror 6 Diff Editor

Adds a side-by-side diff editor using CodeMirror 6, integrated into the existing DiffView component.

Features

• Side-by-side MergeView with syntax highlighting for C#, JS/TS, Python, CSS, HTML, JSON, XML/XAML, Markdown, Bash
• Table/Editor toggle per diff file — switch between existing table view and CodeMirror
• Collapsed unchanged regions (like GitHub PR view)
• Line numbers on both sides with gutter change markers
• Custom dark theme matching PolyPilot's palette
• Search (Ctrl+F), fold gutters, bracket matching
• Pre-built 661KB bundle — works offline, no CDN dependency

Technical Details

• CodeMirror 6 bundle built from codemirror-src/ via esbuild (reproducible)
• DiffParser.ReconstructOriginal/ReconstructModified extract both sides from parsed diffs
• Instance lifecycle managed via JS Map + IAsyncDisposable
• 3 new tests for reconstruct methods
• Also fixes pre-existing ChangeModelAsync test compilation errors (missing named param)

Verified

• Build: 0 errors
• Tests: 3132/3132 pass
• MauiDevFlow: C# syntax highlighting, diff colors, collapsed regions all confirmed via screenshots

[PureWeen]

🔍 Multi-Reviewer Code Review — PR #507 (Re-review #4 — Final)

CodeMirror 6 Side-by-Side Diff Editor

3 independent reviewers re-analyzed this PR after commit f6edbdd6 ("fix: disable Apply without FileContentProvider and fix editor-view renamed-file comments"). CI: ⚠️ No checks configured on this branch.

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Previous Findings — Status

#	Severity	Finding	Status
NEW-1	🟡	Inline chat diffs missing FileContentProvider — Apply writes blank gaps	✅ FIXED
NEW-2	🟡	Editor-view line comment on renamed file uses wrong filename	✅ FIXED

Details

NEW-1 ✅: CanApplyEdit now gates on both conditions: OnApplyEdit.HasDelegate && FileContentProvider is not null. Since ChatMessageItem passes OnApplyEdit but not FileContentProvider, the Apply button is correctly hidden in chat diffs. The review panel passes both, so Apply works only where content can be gap-filled. Confirmed by all 3 reviewers.

NEW-2 ✅: Three-layer fix verified by all 3 reviewers:

• C# InitCodeMirrorForFile passes oldFileName to createMergeView
• JS commentMeta stores oldFileName: oldFileName || filename
• JS getLineClickExtensions selects: side === 'original' ? meta.oldFileName : meta.fileName
• Table view already used OldFileName correctly from the previous round

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Full Finding History (Reviews 1–4)

Round	Finding	Final Status
R1	🔴 Fire-and-forget dispose + sync Clear() leaks JS instances	✅ Fixed (R2)
R1	🔴 Stale InitCodeMirrorForFile writes orphaned ID	✅ Fixed (R2)
R1	🟡 Double-init on rapid toggle	✅ Fixed (R2)
R1	🟡 DisposeCmInstance races with DisposeAsync	✅ Fixed (R2)
R1	🟡 ReconstructOriginal/Modified omit inter-hunk gaps	✅ Fixed (R2)
R1	🟡 Pre-built bundle without integrity verification	⚠️ Partially fixed (R2) — acceptable
R1	🟢 O(n²) Files.IndexOf in render loop	✅ Fixed (R2)
R1	🟢 Bare catch {} blocks	✅ Fixed (R2)
R2	🟡 Post-loop Clear() wipes new instances	✅ Fixed (R2)
R2	🔴 Apply writes blank placeholder lines → data loss	✅ Fixed (R3)
R2	🔴 Path traversal bypass	✅ Fixed (R3)
R2	🟡 Renamed file comment wrong filename (table)	✅ Fixed (R3)
R2	🟡 _applySaveStatus bleeds across files	✅ Fixed (R3)
R2	🟢 Uncached AddedLineCount/RemovedLineCount	✅ Fixed (R3)
R2	🟢 Stale CSS comment	✅ Fixed (R3)
R3	🟡 Chat diffs missing FileContentProvider	✅ Fixed (R4)
R3	🟡 Editor renamed-file comment wrong filename	✅ Fixed (R4)

17 findings across 4 review rounds — all resolved. 🎉

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ℹ️ Discarded Findings (1/3 only, not confirmed)

• simulateLineClick doesn't use oldFileName for original-side — test utility only, not called in production
• Double-logging on Apply write failure — throw in handler propagates to DiffView catch. Cosmetic stderr noise.
• FileContentProvider not plumbed to inline chat diffs — intentional design gap (Apply disabled there via CanApplyEdit guard). Follow-up feature, not a bug.
• JS options null dereference in createMergeView — false positive; JS default parameter options = {} handles null

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Recommendation: ✅ Approve

All findings from all 4 review rounds are resolved. The PR adds a substantial, well-tested feature (CodeMirror 6 diff editor with side-by-side view, syntax highlighting, editable modified side, Apply, line commenting, file picker) with solid lifecycle management (snapshot-before-clear disposal, generation counter, double-init guard, path traversal protection, content-aware gap filling).

Ship it! 🚀