Review Side Rail
The right-hand review panel container. A header plus an APG Tabs region (Comments / Activity / Changes) over a scrollable body. It hosts presentational review content — comment threads, activity feeds, change lists — and collapses to a thin vertical rail with a toggle so the document reclaims the width. Token-driven on var(--surface-sunk); controlled or uncontrolled open state and selected tab. Presentational only — the editor engine lives in the consuming app.
Uncontrolled — collapse with the header chevron, reopen from the rail
Controlled open state — externally driven collapse
Composed children (no declarative tabs)