Highlight changed words within lines in the web and desktop diff views

[not-stbenjam]

Problem

When a diff changes part of a long line, such as a reworded sentence in Markdown or one argument in a function call, the web and desktop diff panels shade the whole removed line red and the whole added line green. The change itself is not marked, so you have to compare the two lines by eye to find it. GitHub, and T3's own mobile review diff, highlight the changed words inside each line pair (an intra-line or word diff), so the edit is visible right away.

Example diff in T3:

Example word-level diff on GitHub:

The latter is much more readable.

Current state

• @pierre/diffs supports this through lineDiffType: 'word-alt' | 'word' | 'char' | 'none'. Its default is 'word-alt', and it skips lines longer than maxLineDiffLength: 1000.
• Web and desktop set it to "none" explicitly:
  • apps/web/src/components/DiffPanel.tsx (thread diff panel)
  • apps/web/src/components/pullRequest/PullRequestCodeTab.tsx (PR code tab)
• That override has been there since the first version of the diff panel (Checkpointing & Diffs #64). I couldn't find a stated reason in the commit, the PR, comments, or docs. The PR code tab copied the same options.
• Mobile already does word diffs. apps/mobile/src/features/review/nativeReviewDiffAdapter.ts computes word-alt ranges for paired removed and added lines, and caps them with NATIVE_REVIEW_MAX_WORD_DIFF_RANGE_COUNT. So the clients are inconsistent today.

Proposal

Switch both web call sites to lineDiffType: "word-alt", which is the library default and matches mobile, and keep the library's maxLineDiffLength limit. Or, make it configurable.

Before changing it, measure render and worker time on a large agent-turn diff with the setting on and off. The likely reason for "none" is performance caution. Parsing has since moved to the diff worker pool, and highlighting now uses the Oniguruma WASM engine (#8360), so that concern may no longer apply. If there is a measurable cost, a setting next to the existing diff layout and line-wrap toggles would be the fallback, but the default should probably be on.

Open question for maintainers: was there a specific reason for turning this off?

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Filed by Claude (Opus 5.5) via Claude Code in T3 Code.