[TUI] Add line+word diff highlighting to the TUI diff viewer

What feature would you like to see?

The Codex CLI TUI diff viewer to support intra-line diff highlighting (word-level or token-level), not just whole-line insert/delete indicators.

Today, any change inside a long line (e.g. adding a single identifier in a long statement, tweaking a string, or small doc edits) shows the entire line as added or removed, which makes it harder to visually scan what actually changed.

Rationale / impact:

• Speeds up code review inside the CLI.
• Reduces cognitive load when scanning large diffs with minor edits.
• Aligns with expectations set by tools like git diff --word-diff, GitHub’s web UI, and popular TUI diff viewers.

Example:

Existing TUI line diff:

<img width="673" height="227" alt="Image" src="https://github.com/user-attachments/assets/405973a7-7df2-4f09-a6a6-276c6e83e3cd" />

Example of improved 'line+word highlight' diff:

<img width="666" height="213" alt="Image" src="https://github.com/user-attachments/assets/765a0699-216e-4593-a3eb-9ec9147b9d1e" />

This example was generated with this diff-strings helper from my dotfiles:

• https://github.com/0xdevalias/dotfiles/blob/eb809aa054a27f72d3009e942f57b6f2df91551b/bin/diff-strings#L58-L66

It basically just does a normal git diff --no-index to pass the 2 strings in; but the real 'line+word' highlighting magic comes from delta:

• https://github.com/dandavison/delta
• > delta
• > A syntax-highlighting pager for git, diff, grep, and blame output
• https://github.com/dandavison/delta#features
• > Features
• > Word-level diff highlighting using a Levenshtein edit inference algorithm

Since delta is also rust-based, maybe the same method / underlying libraries / etc can be used directly?

Are you interested in implementing this feature?

I don’t currently have the Rust familiarity or bandwidth to implement it, so I’m filing this as a feature request for consideration.

Additional information

The following proposed implementation was generated by GitHub Copilot (GPT-5); and I haven't manually validated it for viability:

Proposed capabilities: 1. Inline highlighting mode that emphasizes only the changed spans within modified lines. 2. Support at least word-level granularity (splitting on whitespace and punctuation); optionally fall back to character-level for very long tokens. 3. (Optional) A toggle key in the TUI (e.g. w) to switch between: - Line-only mode (current behavior) - Word-diff mode 4. (Optional) A configuration option (env var or config file) to set the default mode. 5. Sensible styling: - Keep existing + / - line indicators. - Within an inserted line, highlight only the newly added segments (e.g. bold or different background). - For replaced portions, consider showing deletions and insertions side-by-side (current approach) but with finer-grain highlights per changed span. Possible implementation sketch: - When processing a diffy::Line::Delete followed immediately by a diffy::Line::Insert with similar line numbers (a replace), run an intra-line diff algorithm on the pair. - Use a lightweight algorithm: - Tokenize into words (split on Unicode whitespace + punctuation). - Run a Myers or patience diff at the token level (crates: similar, difference, or implement a small LCS). - For each token classified as changed, wrap it in styled spans (ratatui::text::Span with distinct background or color). - Fallback to character-level diff if tokenization yields too few tokens or tokens are huge. - Provide an abstraction like compute_inline_spans(old: &str, new: &str) -> (Vec<Span>, Vec<Span>) returning styled spans for each line variation. - Integrate into push_wrapped_diff_line or a companion function that can accept a pre-tokenized styled Vec<Span> instead of a plain &str. Open questions / nice-to-haves: - Should deletions be shown inline (e.g. strikethrough) on the inserted line, or kept as a separate deleted line? (Initial version can keep current separate-line layout.) - Add a minimal legend/help panel update indicating the toggle key and meaning of inline highlighting. - Option for users with limited color support or accessibility needs to choose underline/bold instead of color changes.

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Tangentially related:

• https://github.com/openai/codex/issues/2378
• https://github.com/openai/codex/issues/2520
• https://github.com/openai/codex/issues/2932