[Feature]: Add git diff mode toggle in the diff panel

[parlali]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/web

Problem or use case

The diff panel currently only shows checkpoint-based turn-by-turn diffs (diff between checkpoint[N-1] and checkpoint[N]). There is no way to see a standard git diff — for example, the working tree diff (git diff + git diff --staged) or a branch-to-branch diff (git diff main...HEAD).

This means:

• Changes made outside the agent (manual edits, other tools, IDE refactors) are invisible until a new turn creates a checkpoint.
• There is no PR-style "diff against base branch" view for reviewing the cumulative result of a session.
• When checkpoint capture is incomplete (see [Bug]: Diff view captures only the first few edits from a turn and shifts the rest to the next turn #1434), there is no fallback to verify what actually changed on disk.

Proposed solution

Add a toggle in the diff panel header (next to the existing stacked/split and word-wrap toggles) that switches between:

1. Turn diffs (current behavior) — checkpoint-to-checkpoint diffs, with per-turn chip selection.
2. Git diff — a live git diff + git diff --staged of the working tree, or optionally a diff against a configurable base ref (e.g. main, origin/main).

When in git diff mode, the turn chip strip could be hidden or replaced with a branch/ref selector.

Why this matters

• Users who interleave manual edits with agent turns currently lose visibility of their own changes until the next agent turn.
• For PR review workflows, seeing the full diff against a base branch in-app avoids switching to a terminal or GitHub.
• It provides a reliability fallback when checkpoint-based diffs are incomplete ([Bug]: Diff view captures only the first few edits from a turn and shifts the rest to the next turn #1434) or unavailable (Diff panel can get stuck on 'Loading checkpoint diff...' when checkpoint is not yet available #318, [bug] Unable to load diffs in diff panel #217).

Smallest useful scope

A single toggle that switches the diff panel from checkpoint mode to showing the output of git diff HEAD (unstaged + staged changes against the latest commit). No branch selector needed in v1.

Alternatives considered

• Using an external terminal or git diff manually — breaks the flow and loses the integrated file tree / syntax-highlighted diff UX.
• The existing "All turns" chip shows cumulative checkpoint diffs, but this is still checkpoint-based and misses changes made outside the agent.

Risks or tradeoffs

• Git diff can be large in repos with many uncommitted changes; may need truncation or lazy loading.
• Need to decide refresh strategy (on-demand vs. polling vs. file-watcher) to keep the git diff view current.
• Two diff modes may add UX complexity; clear labeling and sensible defaults are important.

Examples or references

• VS Code Source Control diff view (working tree diff with inline toggle)
• GitHub PR "Files changed" tab (branch diff)
• Related issues: [Bug]: Diff view captures only the first few edits from a turn and shifts the rest to the next turn #1434 (checkpoint accuracy), [Feature]: manual git snapshots/turns #1472 (manual snapshots), Diff panel can get stuck on 'Loading checkpoint diff...' when checkpoint is not yet available #318 (checkpoint loading stuck)

Contribution

• I would be open to helping implement this.

[itsfrank]

I implemented this (along with a few other QoL changes) in my fork and I have been very happy with the result.

A list of changes I implemented:

1. An icon toggle to switch between git mode and turn/session mode.
2. Collabsible per-file diffs, with a button to collapse/expand all(works in both Git and turn/session mode.
3. Tightened the UI around file diffs so they take up less vertical space.

Here is what it looks like:
New buttons (red: session/turn <> git toggle, blue: expand/collapse all):

Git worktree mode with expanded files

Same with collapsed files

I have used my fork of t3code on a very large production codebase (>10M C++ lines of code). I have found that in such large codebases, the turn based diffs can take over a minute to update and reflect in the app after a turn has complete, while the git worktree pane updates almost instantly. I have been using this for close to 2 weeks and it has been very nice.

My fork is 100% vibe-coded (gpt 5.4), I did not review any of the changes, so I highly doubt the implementation is mergable as-is, but hopefully this is a constructive addition to the conversation.

I am working on extracting the git diff changes form my fork so that I can share a branch with changes limited to this feature. I attempted to merge upstream/main and hit a ton of conflicts so my implementation may additionally be out-of-date with recent architectural changes.

[itsfrank]

Here is the commit that implements the above (changes against main from ~2 weeks ago). I will attempt to rebase against upstream and share a draft PR: 90f58af

[itsfrank]

Created a draft PR here: #1801

It's not small and focused so it will remain as draft for now, I will try to create a minimal PR implementing only the git source change if I can find the time

[D3OXY]

@itsfrank, shall I pick up? If you don't mind. I already have an implementation of this in my fork which is much more targeted.

[itsfrank]

@D3OXY of course!