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 #3)

CodeMirror 6 Side-by-Side Diff Editor

3 independent reviewers re-analyzed this PR after commit 0e1017bc ("fix: address all 6 PR review findings"). CI: ⚠️ No checks configured on this branch.

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Previous Findings — Status (Re-review #2 → #3)

#	Severity	Finding	Status
1	🔴	Apply writes blank placeholder lines to disk → data loss	✅ FIXED
2	🔴	Path traversal bypass (StartsWith without trailing separator)	✅ FIXED
3	🟡	Renamed file comment references wrong filename on original side	⚠️ PARTIALLY FIXED
4	🟡	_applySaveStatus bleeds across file switches	✅ FIXED
5	🟢	Uncached AddedLineCount/RemovedLineCount	✅ FIXED
6	🟢	Stale CSS comment on .cm-merge-revert	✅ FIXED

Details on Fixes

Finding 1 ✅: ReconstructOriginal/Modified now accept optional string[]? fileLines. When provided, inter-hunk gaps are filled with real file content. DiffView has a new FileContentProvider parameter, and the PR review panel wires it to LoadFileContentAsync. The Apply button writes back what the user actually edited — not fabricated blank lines. Both HandleApplyEditAsync and LoadFileContentAsync have matching path-traversal guards. Confirmed by all 3 reviewers.

Finding 2 ✅: Path check now uses:

var normalizedWorkDir = Path.GetFullPath(workDir)
    .TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar)
    + Path.DirectorySeparatorChar;
if (!filePath.StartsWith(normalizedWorkDir, StringComparison.OrdinalIgnoreCase))

Applied consistently in both read and write paths. Confirmed by all 3 reviewers.

Finding 3 ⚠️ PARTIALLY FIXED: Table view now correctly uses OldFileName for original-side line comments on renamed files. However, the editor (CodeMirror) view still passes file.FileName (new name) to createMergeView, where commentMeta.fileName is shared by both sides. Clicking a line on the original side of a renamed file in editor mode still sends the new filename. See NEW-2 below.

Findings 4–6 ✅: All confirmed fixed by all 3 reviewers. SelectFile clears _applySaveStatus, line counts use ??= lazy caching, CSS comment updated.

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New Findings (this re-review)

🟡 NEW-1 — Inline chat diffs missing FileContentProvider — Apply still writes blank gaps

File: ChatMessageItem.razor (DiffView instantiation)
Flagged by: 2/3 reviewers

The PR review panel correctly wires FileContentProvider="LoadFileContentAsync", but inline chat diffs in ChatMessageItem pass OnApplyEdit without FileContentProvider. If a user switches to Editor view on a multi-hunk chat diff and clicks Apply, inter-hunk gaps are still blank — the same data-loss class as the original Finding 1 but on a different code path.

Fix options:

1. Pass a FileContentProvider to chat DiffView instances too (loads from Session.WorkingDirectory)
2. Disable the Apply button when no FileContentProvider is set (safe default)
3. Add a runtime guard: if FileContentProvider is null and file has multi-hunk gaps, refuse to Apply

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🟡 NEW-2 — Editor-view line comment on renamed file still uses new filename (Finding 3 remainder)

File: DiffView.razor (InitCodeMirrorForFile), index.js (line click handler)
Flagged by: 2/3 reviewers

InitCodeMirrorForFile passes file.FileName (new name) to JS as commentMeta.fileName, which is used for both original and modified side clicks. Table view handles this correctly (uses OldFileName on original side), but editor view does not.

Fix: Pass both file.FileName and file.OldFileName ?? file.FileName to createMergeView; use old name for 'original' side clicks in JS.

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

Finding	Reviewer	Reason
_applySaveStatus not cleared on RawDiff change	1/3	Cosmetic edge case — status auto-clears in 2s and Apply button isn't rendered until editor mode re-entered
_prDiffCacheKey length-based collision	1/3	Theoretical — same PR, same diff length, different content is extremely rare
BeginLineComment spurious async / CS1998	1/3	Valid nit but compiler warning only
Path-traversal-blocked Apply shows "✅ Saved"	1/3	HandleApplyEditAsync returns without invoking callback on block — caller doesn't set "saved". Verified as false positive.
ReconstructOriginal gap-fill uses wrong file version	1/3	Display-only (original side read-only), no data path affected

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Recommendation: ⚠️ Request Changes (minor)

5 of 6 previous findings are fully fixed — excellent iteration! 🎉 The two 🔴 CRITICAL issues (data loss + path traversal) are resolved.

Two 🟡 MODERATE items remain:

1. NEW-1: Disable Apply (or pass FileContentProvider) on inline chat diffs to prevent the same data-loss bug on that code path
2. NEW-2 / Finding 3 remainder: Pass OldFileName to JS for editor-view renamed-file comments

Neither is a show-stopper, but both are real user-facing bugs that will be hit in normal usage. A quick follow-up commit addressing these two would earn a ✅ Approve.