/** Distance from the true bottom at which a downward scroll resumes following. */
export declare const BOTTOM_RESUME_THRESHOLD_PX = 8;
/** Ignore sub-pixel scroll jitter when classifying upward movement. */
export declare const SCROLL_UP_TOLERANCE_PX = 2;
export interface ThreadFollowState {
    readonly following: boolean;
    readonly lastScrollTop: number | null;
    readonly lastDistanceFromBottom: number | null;
}
export type ThreadFollowEvent = {
    type: 'scrolled';
    scrollTop: number;
    distanceFromBottom: number;
} | {
    type: 'scrolled-up';
} | {
    type: 'bottom-requested';
} | {
    type: 'reset';
};
export declare function createThreadFollowState(): ThreadFollowState;
/**
 * Keeps streaming layout changes separate from deliberate reader scrolling.
 *
 * `scrollTop` alone cannot distinguish a reader scrolling up from layout
 * moving the content under a stationary reader: a collapsing live-activity
 * box shrinks `scrollHeight` and the browser clamps `scrollTop` down, and
 * `maintainVisibleContentPosition` adjusts `scrollTop` when a row above the
 * viewport re-measures smaller. Both look like an upward scroll but keep the
 * reader the same distance from the bottom (or closer). A *real* upward
 * scroll moves the reader **away** from the bottom, so detaching requires
 * both signals: `scrollTop` decreased *and* `distanceFromBottom` increased.
 */
export declare function reduceThreadFollow(state: ThreadFollowState, event: ThreadFollowEvent): ThreadFollowState;
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