perf: diff pipeline causes severe UI lag on desktop — full-context patches + synchronous parsing on main thread #22650

Description

The diff pipeline (both VCS diff and Session diff) causes severe UI jank on the desktop app. Disabling both systems completely eliminates the lag — confirming the diff subsystem as the root cause.

This affects two independent diff systems:

• VCS diff (/vcs/diff endpoint) — git working tree / branch diff
• Session diff (SessionSummary) — AI-generated file change diffs per session

Root Cause Analysis

The core design issue is using full-context unified patches as the universal transport format between backend and frontend, which creates triple redundant work:

1. Backend generates full-file patches with formatPatch(structuredPatch(..., { context: Number.MAX_SAFE_INTEGER })) — expensive git ops + string processing
2. Frontend parses the patch back into before/after content via parsePatch() in session-diff.ts
3. Frontend feeds reconstructed content to @pierre/diffs parseDiffFromFile() which re-diffs it — synchronously on the main thread

Bottleneck Ranking (by perceived lag contribution)

Suggested Fix Strategy

1. Split diff API into summary / detail

• Summary (list view): return only { file, status, additions, deletions } — no patches
• Detail (per-file, on expand): return { before, after, status } for a single file
• Both VCS and Session diff share this contract

2. Lazy normalize in SessionReview

• Stop eagerly calling normalize() for all files in items()
• Only compute FileDiffMetadata when a file is expanded && visible

3. Move parseDiffFromFile() to web worker

• Current worker pool (size=2) only handles Shiki highlighting
• Diff parsing should also be offloaded from the main thread

4. State granularity + debounce

• Split session_diff store into summaryBySession / detailByKey
• Add 150–250ms debounce to SSE diff event handling
• Replace rAF + getBoundingClientRect() scroll tracking with IntersectionObserver

5. Backend: cache + reduce redundancy

• Cache Snapshot.diffFull results by (from, to) snapshot pair (immutable)
• Separate read lock from write lock on snapshot semaphore
• Combine the two sequential git diff calls (--name-status + --numstat) where possible

6. Remove Number.MAX_SAFE_INTEGER from interactive path

• Warning: cannot simply change to -U3 without updating the client — session-diff.ts currently depends on full-context patch to reconstruct before/after content
• For export/share use cases, generate full patches via a separate background path

Related Issues

• VCS diff endpoint returns HTML instead of JSON causing TypeError in Web UI #19291 — VCS diff endpoint returns HTML instead of JSON with git submodules (amplifies lag via retry loops)
• Diff endlessly reloading in web app #22625 — Diff endlessly reloading in web app (same submodule root cause)

Environment

• OpenCode version: 1.4.x
• OS: macOS (Apple Silicon)
• Client: Desktop app
• Repo size: medium-large with git submodules