Choose right-panel surfaces that open automatically in every thread

[coygeek]

Summary

Add a setting where the user picks right-panel surfaces, such as Agents, that open automatically in every thread. A user who always works with a surface open should not have to reopen it thread by thread.

Problem to solve

Right-panel state is stored per thread, and a thread with no saved panel state starts with the panel closed and empty. A user who always keeps the Agents surface open has to open it again in every new thread: click the right-panel toggle in the header, then choose Agents from the "Open a surface" menu.

The menu's letter accelerators, such as A for Agents, only work while the menu is open, and no command opens the menu or a specific surface directly (see #11878). So there is no one-step way to open Agents, and no way to make the choice stick. For someone running subagents across many threads, this happens many times a day.

Proposed behavior

• Add a setting to the "New threads" section of Settings, next to Model, Permissions, and Workspace. In it the user picks zero or more thread surfaces (Agents, Terminal, Files, Diff, Browser, Linked pull requests, Device) and which of them is active.
• Scope it like the other "New threads" settings: one value for All projects, which a project can override.
• When the user opens a thread that has no saved right-panel state, the panel opens with the chosen surfaces and the chosen surface active. A surface the thread cannot show yet, such as Diff in a draft or Linked pull requests when the thread has none, is skipped quietly.
• Once the user changes the panel in a thread (closes it, closes a surface, or adds one), that thread keeps the user's choice. The default never reopens something the user closed in that thread.
• When the right panel appears as a sheet over the chat (narrow windows), the default does not open that sheet automatically. Proactive panels already stays off in that layout.
• An empty selection, the default, keeps today's behavior.

Acceptance criteria

• With Agents selected, creating a new thread shows the right panel with Agents active, with no clicks.
• Switching to an existing thread that has no saved panel state also shows the panel with Agents active.
• Closing the panel in Thread A, switching away, and switching back leaves Thread A's panel closed. Other threads still get the default.
• A project override replaces the All projects value for threads in that project. Removing the override brings back the All projects value.
• With nothing selected, new threads start with the panel closed, exactly as they do now.
• In the narrow sheet layout, opening a new thread does not cover the chat with the panel.
• The setting and its project overrides persist across app restarts.
• With Proactive panels on, a qualifying thread still gets its linked pull request or diff tab alongside the default surfaces.

Affected area

apps/web: the right-panel surface state (apps/web/src/rightPanelStore.ts, where each thread starts from a closed, empty state), the "New threads" settings with project overrides (apps/web/src/components/settings/ProjectDefaultsSettings.tsx), and the settings schema in packages/contracts.

Non-goals

• Keybinding commands that open individual surfaces. Keybinding commands to open the pull request and terminal tabs in the right panel #11878 already tracks those.
• One panel shared across threads. The panel stays thread-scoped, so per-thread tabs like terminals and open files keep working as they do.
• Panel width, tab order, or other layout changes.

Alternatives considered

• Keyboard route (open the panel, open the "Open a surface" menu, press A). It still costs several steps per thread and never sticks.
• [Feature]: Add per-project default right panel tabs for new threads #6829 (per-project default of Diff and Files) and [Feature]: Global default panel layout for new project chats (files/terminal auto-open) #6906 (global auto-open of Files and Terminal) both describe the same root problem with a fixed preset. A user-chosen set, stored for All projects with project overrides, would satisfy both and also cover Agents and any other thread surface.

Supporting context

Observed on T3 Code desktop 0.0.43-nightly.20260927.2365 on macOS. Checked against current main: a thread with no saved panel state starts closed and empty, the Agents surface can only be opened from a thread, and the letter accelerators in the surface menu work only while that menu is open.

[MarcelDuchamp]

+1. I'd like the right panel open in every new thread. Today I press ⌘⌥B in each one, many times a day.

This is still the case on T3 Code (Beta) 0.1.0 on macOS: a thread with no saved right-panel state starts closed, and nothing carries the state over from the previous thread. Proactive panels doesn't cover it. It opens a panel only after a turn, for a linked pull request or a larger edit, never when the thread is created.

The surface picker proposed here would fully cover my case. If that's too big for a first step, a single "Open the right panel in new threads" toggle under New threads would already help. #16825 asks for the same default.