SPIKE: Git graph view in the code review app

Date: 2026-06-18 Status: Backlogged — explored, not building now.

What we explored

Idea: a git-graph view in the review app. Show a commit graph; click a commit and its file changes load on the right — basically a cross between the all-files view and a way to switch between different commits' diffs.

This is a research note, not a commitment. We're parking it.

Key finding: the diff viewer is already source-agnostic

App.tsx fetches /api/diff → gets a rawPatch string → parseDiffToFiles() → feeds the FileTree + all-files view + single-file view. The viewer doesn't care where the patch came from. So "click a commit → see its files" = fetch that commit's patch and feed the same pipeline.

The Pierre diff viewer needs ~no changes. That was the main worry, and it's the part that's basically free.

What's already reusable

• runGitDiff (packages/shared/review-core.ts) already does diff modes: uncommitted / staged / last-commit / merge-base / all / branch. last-commit is literally git diff HEAD~1..HEAD — so a single commit's diff (git diff <sha>^..<sha>) is a tiny new mode.
• listRecentCommits() (review-core.ts ~245) already runs one git log --pretty and returns RecentCommit[] (powers the base-branch picker). That's the seed for a commit list.
• /api/diff/switch already swaps the active diff and re-renders. Selecting a commit is conceptually another switch.

What's net-new

1. Per-commit diff — extend the diff backend to take a sha (git show / git diff <sha>^..<sha>); context-expansion base becomes the parent. Small. Must be mirrored in the Pi server.
2. Graph data — add parent hashes (%P) + refs (%D) to the log, then compute lane layout (which column each dot/edge sits in). The DAG parse is easy; the lane/rail layout is the fiddly part.
3. Graph UI — rows + SVG rails (dots, colored branch lines), commit metadata, selection, virtualization for big repos. This is the bulk of the work.

The real gotcha

The diff system also supports jj and Perforce, not just git. A commit graph is git-specific (jj has its own log/op-log model). So this would be git-only unless we later add a jj variant. Decide up front.

Suggested phasing (if/when we pick this up)

• Phase 1 — commit browser (small/medium): flat commit list in the sidebar
  • per-commit diff mode + click loads into the existing all-files view. No viewer changes, modest backend add. Validates the workflow cheaply.
• Phase 2 — the graph (medium, mostly frontend): layer lane rails / branch colors on top of Phase 1's data + wiring. The "cool visualization" lives here.

Takeaway

Very doable. Phase 1 is a surprisingly small lift because the viewer is decoupled. Backlogged for now — revisit when there's appetite, start with Phase 1.