[Feature Request] Per-message diff scope in the Desktop diff panel

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The Desktop app's diff panel currently shows the cumulative diff for the entire session. In long sessions where I send multiple prompts and Claude makes edits across many turns, I lose visibility into what changed in response to my most recent message.

Today, to answer "what did Claude just change?" I have to either:

• Mentally subtract earlier changes from the session-wide diff, or
• Drop to the terminal and run git diff against an ad-hoc reference point I tracked manually.

This breaks the review flow, especially when iterating on a single feature over many turns.

Proposed Solution

Add a scope toggle to the diff panel:

1. Session (current behavior) — diff of all changes since session start.
2. Since last message (new) — diff of only the changes Claude made in response to the most recent user message.
3. Since message N (stretch) — let users pick any prior user message in the transcript as the diff baseline.

Under the hood this just means snapshotting the working-tree state at each UserPromptSubmit boundary and diffing against the chosen snapshot.

UI sketch: a small dropdown or segmented control at the top of the existing diff panel - "Scope: [Session ▾]" with options: Session / Last message / Pick message….

Alternative Solutions

• Manual git stash create via a hook on every prompt submit, then git diff against the stash. Works, but requires a clean git state and lives outside the Claude Code UI.
• Per-prompt rewind (#43755) is related but different — it's about reverting, not reviewing.

Priority

High - Significant impact on productivity

Feature Category

Developer tools/SDK

Use Case Example

1. I ask Claude to scaffold a feature; it edits 6 files. I review and accept.
2. I send a follow-up: "now add tests for the error path." Claude edits 3 files.
3. Today: the diff panel shows all 9 files mixed together.
4. With this feature: I switch scope to _Since last_ message and immediately see only the 3 test-related changes — the exact scope of my last instruction.

Additional Context

Related issues:

• #33932 — VS Code diff review UI (different surface, related theme)
• #48744 — Expand/Collapse all in Desktop diff panel
• #43755 — Per-prompt rewind/undo