Shell plugin: three-column AppFrame (drag handles and concession chain) plus the ctx.layout panel-geometry service; it registers into the runtime-owned root slot and declares sidebar, conversation, details, and conversation.empty. The sidebar is fixed-width (only details shrinks, then auto-closes); a closed sidebar retains a 56px control rail while details closes to zero width. The package also seats the theme presenter: it consumes resolved ctx.theme snapshots and projects them onto the document (html { color-scheme } for native UA chrome, body[data-ds-dark-theme] from the active color scheme, plus the theme's alias tokens as inline variables on body).

AppFrame always mounts the conversation and details columns; a connected Session renders through SessionProvider. The transient layout store starts both panels at their default widths and never reads or writes localStorage. Hero and other unselected states derive a zero rendered details width without changing that stored preference. AppFrame retains the last non-blank Session id across those states: the first Session opens at the default width, returning to the same Session restores its unchanged width, and selecting a different Session closes details before paint. The conversation owner share is empty, while the sidebar owner share contains only collapsed and width; registrants obtain business data from standard hooks and actions from their own inject faces.

The /client export surface is the plugin body (apply/inject), LayoutService, and the four owner-share interfaces. AppFrame, the panel store, and the concession solver remain package-internal; tests import internals through /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

Panel geometry is transient — reload restores both panels to their defaults; switching between distinct Session ids closes details and forgets its dragged width, while unselected surfaces render details at zero width without modifying geometry.
Concession-chain auto-close derives a zero width without touching the preferred width — the panel restores itself when the window widens; consumers must not read the stored details width as the rendered truth.
Scroll anchoring during squeeze reflow is not implemented — deferred with the virtualized-list project.