[FEATURE]: TUI: timestamp gutter mode for messages #29399

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

This specific proposal — a tri-state /timestamps cycle (hide → footer → gutter → hide) with a fixed HH:MM left gutter on both user and assistant messages and a click-to-popup for the full second-precision date — is new. Several adjacent requests exist and are cited under "Prior demand" below; this issue offers a concrete design and PR that addresses several of them.

Describe the enhancement you want to request

The TUI already has /timestamps to show the time under each user message ("footer" mode). When scanning a long conversation it's useful to see when each message was sent at a glance, but the footer style takes a full line per message, doesn't align across rows, and only applies to user messages.

Proposal

Extend /timestamps to cycle three modes instead of two:

• hide (current default) — nothing shown
• footer — current "show" behavior, time on a line below each user message
• gutter — fixed 5-char HH:MM column to the left of each message (user and assistant), 24-hour. Click the gutter to open a small popup with the full local datetime (HH:MM:SS) and relative time

Add an optional tui.json key timestamps_mode ("hide" | "footer" | "gutter", default "hide") that seeds the default for new users. The slash command still cycles at runtime; the config key just sets the starting mode. Existing users keep their current toggle — the kv-stored legacy "show" value is normalized to "footer" at read time.

Prior demand for this feature — and what this PR addresses

Closed by this proposal:

• [FEATURE]: Add timestamp next to messages in chat (both agent and user messages) #8634 — [FEATURE]: Add timestamp next to messages in chat (both agent and user messages) (8 👍 + 1 ❤️, 3 supporting user comments describing the exact use case: spotting stalled overnight sessions, detecting dropped sub-sessions, finding stale parallel chats). The gutter renders on both user and assistant messages.
• [FEATURE]: Add seconds to message timestamp display #20406 — Add seconds to message timestamp display. The popup uses HH:MM:SS.
• [FEATURE]: Add current hour in end of request #25727 — Add current hour in end of request. The gutter shows the message's hour adjacent to the existing ▣ Mode · model · duration footer line.
• /timestamps command has no visible effect and /exit missing from autocomplete #26625 — Bug report that the existing /timestamps has no visible effect. The new gutter mode is visibly different by construction; whatever rendering state the footer mode is in today, users always have a working alternative.
• Timestamps always show AM/PM on Linux (Bun ignores system locale) #28804 — Timestamps always show AM/PM on Linux (Bun ignores system locale). The gutter and popup both format 24-hour explicitly via manual getHours()/getMinutes()/getSeconds() rather than going through Bun's locale path, side-stepping that bug for the new code paths.

BTW, even looks like someone is maintainig their own fork with this feature, so people do want this!
ariane-emory#191

Why this design

Three alternatives considered:

• A (this proposal): one cycle, three modes. Single mental model. One slash command. Config seeds default.
• B: independent toggles (/timestamps for footer, /timestamp-gutter for gutter). Both can be on at once, duplicates the timestamp visually.
• C: replace footer with gutter behind the same toggle. Cleaner visually but breaks current users' UX without a way back.

Scope deliberately not in v1:

• Mouse-only popup. Keyboard support (t on a selected message) needs a new selectedMessageID signal — follow-up PR.
• Gutter width is hardcoded to 6 cells (HH:MM + space). No dynamic measurement, keeps alignment stable across narrow and wide terminals.

PR

PR #29398 has the implementation, docs, and unit tests (14 new tests, all green, typecheck clean across all 15 packages).