import { type RefObject } from "react";
/** A line's vertical extent, in px. */
export interface LineBox {
    top: number;
    bottom: number;
}
/**
 * A VALUE'S FIRST LINE, whatever it draws: its topmost line of words — a text's line box, a control's own line, a
 * switch, a meter's track — unless a row of pictures stands wholly above every word (a grid of photos over its add),
 * which is then its first line. A picture beside words (a face by its name, a thumbnail by its title) never is: the
 * words are the line. None where it draws neither.
 */
export declare function pickFirstLine(words: readonly LineBox[], pictures: readonly LineBox[]): LineBox | undefined;
/**
 * Where the first line of what `ref` draws centres, in px from its top — read off the rendered boxes before the
 * first paint and again whenever what it draws changes size or content; null where it draws no line, or `on` is
 * false, and without a layout engine (jsdom), where nothing is measured.
 */
export declare function useFirstLine(ref: RefObject<HTMLElement | null>, on: boolean): number | null;