feat(web): open a message as a right panel surface

[rt2zz]

What Changed

Adds a "open message as a surface" feature. Exposed as a button next to the copy message button.

Why

Often the model gives a long report. I want to respond to one point immediately while continuing to formulate a response to the rest. Currently I copy the message into zed to do this, but way better if I can just pop it open as a surface.

I kept this intentionally minimal using only existing concepts

• Does not support attachments yet
• I don't actually think this deserves a standalone button, it should either be in a context menu or overflow menu, and most importantly bindable to a shortcut. I did not add a shortcut though because that would depend on a concept of message focus which would be independently valuable, but require more significant and mostly orthogonal changes.

future ideas:

• give message surface tabs useful titles
• path to get back to a conversation (or fork) from a message surface

UI Changes

Before:

After:

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• [n/a] I included a video for animation/interaction changes

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Note

Low Risk
UI-only extension of the existing right-panel surface pattern; no auth, API, or persistence schema beyond the versioned panel store already used for other surfaces.

Overview
Users can open a chat message in the right panel as a dedicated surface, alongside files, diff, and terminal—useful for reading long assistant replies while composing a response in the main thread.

Right panel model: Adds a message surface kind and openMessage(ref, messageId) in rightPanelStore, with per-message tab ids (message:messageId) and persistence bump. UI: Timeline rows expose an Open as surface control (panel icon) next to copy on user and assistant messages; ChatView renders MessageSurface when that tab is active, resolving the message from timeline entries and showing an empty state if it was removed. Tabs: RightPanelTabs labels message surfaces as "Message" with a MessageSquare icon.

User message text in the surface uses the same displayed text as the timeline (terminal context stripped via deriveDisplayedUserMessageState).

^{Reviewed by Cursor Bugbot for commit 21a5706. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add message surfaces to the right panel in the chat view

• Adds a message surface kind to rightPanelStore that stores a message by ID, deduplicating on repeated opens and setting it as the active surface.
• Adds OpenMessageSurfaceButton (panel-right icon) to user and assistant message action bars in MessagesTimeline, wiring to openMessage in the store.
• Adds MessageSurface component that renders the message content as markdown in the right panel, with a 'Message unavailable' fallback for missing messages.
• Updates RightPanelTabs to show a message-bubble icon and 'Message' title for message surface tabs.

^{Macroscope summarized 21a5706.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new user-facing feature (opening messages in the right panel surface), adding new components and extending the panel store with a new surface type. New capabilities warrant human review even when implementation follows existing patterns.

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[rt2zz]

FYI: I removed attachment support, not because its a bad idea but because it implies a slighly larger component refactor and I didnt want to dilute the main suggestion here unless theres a baseline interest