Shell plugin: three-column AppFrame (drag handles, concession chain) + ctx.layout viewing-state service (nav, panel widths, persist); defines the sidebar/conversation/details/conversation.empty slots. The sidebar is fixed-width (it never concedes to viewport pressure — only details shrinks, then auto-closes); a closed sidebar retains a 56px control rail while details closes to zero width; collapse/expand animates the grid tracks on the deepsuite sider curve. Contract: api-contracts v3 §5.

Slot declarations use the composed-props entry form (owner share, no full props): the exported OwnerShare contracts are SidebarOwnerProps / ConvOwnerProps / DetailsOwnerProps / EmptyOwnerProps — registrants reference them via OwnerOf<'sidebar' | ...> and compose their own injected share locally. No entry declares children (declaring it requires the registered component to carry the slots face — reserved for future business slots): delegation authority is the component-side whitelist, i.e. AppFrame's ScopedSlots<FrameSlotKey> face over sidebar/conversation/details/conversation.empty. Since the root-slot rework the frame itself registers into 'root' and renders those child slots at its own render sites; the shell only renders 'root'.

The export surface is the cross-package contract only: the AppFrame trio (+ AppFrameProps) consumed by the web shell's assembly, LayoutService with its store shapes (NavState/PanelState/ViewId), and the OwnerShare contracts. The concession-chain solver (computeColumns) and its geometry constants are package-internal; tests import them from /src.

Model Experience

None, as the layout shell manages browser viewing state; nothing here reaches a model request.

KV Cache effect

None; this package neither assembles nor sends a provider request.

Known Limitations and Deferred Work

Details open/width state is global — it does not follow the session (arbitrated for P-I); the per-session keyed upgrade slot is reserved.
Concession-chain auto-close derives a zero width without touching the persisted open flag — the panel restores itself when the window widens; consumers must not read details.open as the rendered truth.
Scroll anchoring during squeeze reflow is not implemented — deferred with the virtualized-list project.