Keyboard review for the Diff panel and the pull request Code tab

[cwooldridge1]

Problem

Reviewing a diff in T3 Code needs the mouse. In the Diff panel and in a pull request's Code tab, you can't step through files, mark a file viewed, find the next file you haven't read, or comment on a line from the keyboard. On a large agent change this is slow, and it's easy to lose track of what you've already read.

Proposal

Make both diff views reviewable from the keyboard. Keys work while focus is in the diff's panel and never while you type in a text field. There are three levels, and Esc steps back one.

Level	Keys
Files (start here)	j/k next or previous file. n/Shift+N next or previous unviewed file. v toggle viewed, which also folds. Enter fold or unfold. f show one file at a time. i move into the file's lines.
Lines	j/k move one line, leaving the file at either end. Shift+J/Shift+K show more unchanged lines. c comment. Shift+V start a selection. v mark viewed and return to files.
Selection	j/k grow or shrink. c comments on the selected lines.

Screen.Recording.2026-11-02.at.2.50.48.PM.mov

Supporting pieces:

• Every key is a remappable diff.* keybinding command, behind new diffFocus and diffLineMode when-clauses.
• Each file header gets a Viewed checkbox and the diff shows an N / M viewed count. In the Code tab these are GitHub's Viewed marks. In the Diff panel they are stored per device and thread, and a file that changes reads as unviewed again.
• An optional setting, off by default, moves to the next unviewed file after you mark one viewed.

Out of scope: mobile, syncing Diff panel viewed marks across devices, and per-hunk viewed state.

Working implementation

I've built this in #28917 to check that it's feasible and fast. Cursor moves don't re-render the diff, and stepping through files scrolls only as far as it needs to. Please treat it as a prototype for this discussion, not a request to merge as is. I'll reshape or split it however you want.

Questions for maintainers

1. Do you want keyboard review in the Diff panel and the Code tab?
2. Are single-letter defaults acceptable while the diff has focus, or should they be unbound by default?
3. Is a per-device viewed store acceptable for the Diff panel, or should viewed marks stay a pull-request-only feature?
4. Should this land as one PR, or split? One possible split is file-level keys with viewed state first, then line mode and comments.