Overflow

Headless priority-overflow system — the core mechanism behind a single-line Fluent 2 Ribbon. A row of controls that cannot fit hides its lowest-priority items into a '…' overflow menu and restores them (highest-priority first) as it grows. Ships a framework-agnostic manager (createOverflowManager) plus React bindings: Overflow provider, OverflowItem, OverflowDivider, and the useOverflowMenu / useIsOverflowItemVisible / useIsOverflowGroupVisible / useOverflowCount hooks. ResizeObserver-driven, priority + pinned ranking with DOM-position tie-break, size caching, group tracking, and a referentially-stable subscribe/getSnapshot store built for useSyncExternalStore. SSR-safe (everything visible until measured) and headless (no styling of its own).