/** Scroll metrics of the viewport at one point in time. */
export interface ScrollSnapshot {
    scrollTop: number;
    scrollHeight: number;
    clientHeight: number;
}
/**
 * Distance from the bottom edge within which the user counts as "at the
 * bottom": entering it re-sticks, and tiny upward wiggles inside it (trackpad
 * inertia, macOS bounce) never unstick.
 */
export declare const RESTICK_PX = 24;
export declare function distanceFromBottom(snapshot: ScrollSnapshot): number;
/**
 * Classify one scroll event. `lastScrollTop` is the previous event's position.
 * Order matters: the bottom zone wins over direction, so bounce-back at the
 * bottom edge stays stuck.
 */
export declare function resolveScrollIntent(snapshot: ScrollSnapshot, lastScrollTop: number): 'stick' | 'unstick' | 'none';
/**
 * How the message list changed between two renders, judged purely from ids —
 * content growth inside an existing message (streaming) is deliberately NOT a
 * transition (the follow behaviour reacts to it via ResizeObserver instead).
 *
 * - `append`   — new messages at the end (feeds the new-messages counter).
 * - `prepend`  — older messages loaded at the top (needs the scroll anchor fix).
 * - `truncate` — trailing messages removed (stable head, shorter list): NOT an
 *   arrival, so it must never bump the counter (review finding, P2 wave).
 * - `replace`  — both ends changed or the list was reset (jump to bottom).
 * - `initial`  — first non-empty render (open at the bottom).
 * Simultaneous append+prepend degrades to `replace` — rare (history load
 * during streaming) and safe: worst case is one jump to the bottom.
 */
export type ListTransition = 'none' | 'initial' | 'append' | 'prepend' | 'truncate' | 'replace';
export interface ListIdSnapshot {
    firstId: string | undefined;
    lastId: string | undefined;
    length: number;
}
export declare function classifyTransition(prev: ListIdSnapshot, next: ListIdSnapshot): ListTransition;
/**
 * New messages added by an `append` transition. A same-length append (the tail
 * message was swapped for a new id) still counts as one new message.
 */
export declare function appendedCount(prevLength: number, nextLength: number): number;