[Feature]: Keep terminal visible beside the turn diff while a run is active

[akshatsha5ill]

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

When an agent turn finishes, I need two truths at once: what changed (turn diff in DiffPanel) and whether it works (ThreadTerminalDrawer output / preview). Today the right panel is single-active (activeSurfaceId in rightPanelStore.ts:85), so opening the diff hides the terminal and I tab-switch terminal ↔ diff ↔ preview to confirm every change. The diff says what moved; only the running output says it passes.

Proposed solution

Allow two existing right-panel surfaces to stay visible side-by-side, per thread, behind a default-off toggle. No new surfaces, no new backend, no workspace mode.

Original rough mock (boxes only, not from any product):

TODAY — right panel shows one surface at a time:

+-----------+-------------------------+
| thread | turn diff |
| chat | M src/auth.ts +12 -3 |
| | (terminal hidden |
| | behind its tab) |
+-----------+-------------------------+

PROPOSED — terminal pinned beside the diff:

+-----------+----------------+--------------+
| thread | terminal [*] | turn diff |
| chat | $ npm run dev | M src/auth.ts|
| | PASS 18/18 | +12 -3 |
+-----------+----------------+--------------+

Legend:

• Left column = existing chat column, unchanged.
• Middle = existing terminal surface, kept visible by a new
  per-thread pin toggle (default off) in the terminal tab header.
• Right = existing DiffPanel in embedded mode, unchanged component.
• Closing either surface falls back to today's single-active tabs.

States

• Pinned layout survives reconnect/reload (persisted per-thread key, versioned like t3code:right-panel-state).
• No extra WebSocket traffic: reuses existing subscribeTerminalEvents + getTurnDiff streams.
• No continuously repainting layout (per the performance rule in AGENTS.md).

Why this matters

A diff without its running output can't answer "does it work?" Tiled agent-workspace tools show provider output beside the change summary for exactly this reason. This proposes only the minimal T3 version of that pattern: two surfaces we already have, visible together.

Smallest useful scope

• P1: pin terminal beside diff only. No preview tiling, no provider-PTY changes.
• Web only (desktop inherits it since it wraps web); mobile explicitly out of scope.
• No packages/contracts change; no new vocabulary in the glossary.
• Default off, so current behavior is untouched unless opted in.

Alternatives considered

• Status quo tab-switching — works, but loses the output at the exact moment of review.
• [Feature]: First-class interop/sync between T3 threads and local Claude Code CLI sessions #6994 (first-class Claude CLI session interop/sync) — server-side session import/sync; explicitly out of scope here. This proposal changes no session behavior.
• A full Agent/Code/Chat workspace mode with a tiled grid — deliberately not proposed; that's a product-direction change and belongs nowhere near a first PR.

Risks or tradeoffs

• Layout persistence needs a storage version bump; old state must fall back to single-active.
• Remote/relay connections must behave the same as local (same surfaces, same streams).
• Narrow windows need a sane fallback (e.g. pin collapses to tabs below a width breakpoint).
• Must not regress single-active performance: no extra subscriptions when the pin is off.

Relevant code context

• apps/web/src/rightPanelStore.ts:28-85 (RightPanelSurface, single activeSurfaceId)
• apps/web/src/components/ChatView.tsx:7506-7542 (exclusive preview ? ... : terminal ? ... : diff ? ... render chain)
• apps/web/src/components/RightPanelTabs.tsx:76-79 (surface tab switching)

Contribution

• I would be open to helping implement this (P1 scope above, size:M, with before/after screenshots).