/**
 * Geometry snapshot for a scrollable chat viewport.
 */
export type ScrollGeometry = {
    scrollTop: number;
    scrollHeight: number;
    clientHeight: number;
};
/**
 * The decision a scroll event implies for follow-bottom state.
 */
export type FollowBottomScrollDecision = {
    nextFollowingBottom: boolean;
    shouldEndLayoutPreservation: boolean;
    shouldScheduleSnapToBottom: boolean;
};
export type MovementAttributionArgs = {
    userScrolling: boolean;
    preservingLayout: boolean;
    /**
     * Whether the movement landed on a clamp boundary (scrollTop 0 or
     * maxScroll). Browser-driven movement during layout turbulence is always
     * a clamp, and clamps can only land on boundaries — so a mid-list
     * landing is user movement no matter what else is in flight.
     */
    landedOnClampBoundary: boolean;
};
export type DecideFollowBottomAfterScrollArgs = MovementAttributionArgs & {
    atBottom: boolean;
    wasFollowingBottom: boolean;
    /**
     * Whether this event moved the viewport upward. Arrivals in the
     * at-bottom zone only re-engage follow when they are not upward: you
     * can only genuinely return to the bottom by moving down (or staying
     * put), while an upward step that happens to land inside the threshold
     * — e.g. a 40px ArrowUp against the 48px default — must not be
     * re-captured after the caller deliberately disengaged (#50).
     */
    movedUp: boolean;
    followBottomThresholdPx: number;
    /**
     * Cumulative upward scrollTop travel (px) since the last non-upward
     * arrival at the bottom, counting only user-attributable movement — see
     * `accumulateUpwardTravel`, which owns the transition rule. This — not
     * the distance from the bottom — is the user's actual displacement:
     * content growth can manufacture an arbitrarily large bottom gap out of
     * a trivial (or nonexistent) gesture.
     */
    upwardTravelPx: number;
};
export type AccumulateUpwardTravelArgs = MovementAttributionArgs & {
    previousScrollTop: number;
    scrollTop: number;
    atBottom: boolean;
    upwardTravelPx: number;
};
export declare const getMaxScroll: (geometry: Pick<ScrollGeometry, "scrollHeight" | "clientHeight">) => number;
export declare const isViewportAtBottom: (geometry: ScrollGeometry, followBottomThresholdPx: number) => boolean;
/**
 * Attribute scroll movement to the user when they are actually giving
 * input, when no layout change is in flight, or when the movement landed
 * somewhere only a user could put it.
 *
 * During layout turbulence the browser moves scrollTop on its own — e.g. a
 * snap write bouncing through a momentarily zero-scrollable boundary while
 * a CSS transition grows a message — and that must not read as "the user
 * scrolled". But those browser moves are clamps, and clamps can only land
 * on a boundary (0 or maxScroll): a mid-list landing during turbulence is
 * programmatic user navigation (scrollTo/scrollBy without input events) and
 * must not be yanked back by follow-bottom.
 *
 * The caller accumulating `upwardTravelPx` and this module's unfollow
 * decision must share this predicate, so it lives here, once.
 */
export declare const isMovementAttributableToUser: ({ userScrolling, preservingLayout, landedOnClampBoundary }: MovementAttributionArgs) => boolean;
/**
 * Advance the upward-travel accumulator for one scroll event.
 *
 * Upward user-attributable movement always accumulates, inside or outside
 * the follow threshold: resetting within the threshold let the component's
 * own snap-backs erase the trail, so a sustained slow programmatic scroll
 * (no scroll intent to suppress the snap) could never accumulate past the
 * threshold and unfollow — a livelock at the bottom (#45).
 *
 * Only a non-upward arrival at the bottom wipes the slate: the component's
 * snap-backs, downward user scrolls, and clamps. Layout-turbulence movement
 * (see `isMovementAttributableToUser`) never accumulates.
 */
export declare const accumulateUpwardTravel: (args: AccumulateUpwardTravelArgs) => number;
export declare const decideFollowBottomAfterScroll: (args: DecideFollowBottomAfterScrollArgs) => FollowBottomScrollDecision;