import { type ChatHeightCache } from './chatMeasurement.svelte.js';
import type { ScrollAnchor } from './chatTypes.js';
/**
 * Capture a scroll anchor before a history prepend operation.
 *
 * Records the first visible message and its pixel offset from the
 * viewport top, so the same visual position can be restored after
 * new messages are inserted above.
 *
 * @param messages - Current array of message objects
 * @param getMessageId - Function to extract a unique ID from a message
 * @param heightCache - The reactive height cache instance
 * @param estimatedHeight - Fallback height in pixels for unmeasured messages
 * @param scrollTop - Current scrollTop of the viewport
 * @returns A scroll anchor object, or null if messages is empty
 *
 * @example
 * ```ts
 * const anchor = captureScrollAnchor(messages, (m) => m.id, cache, 72, viewport.scrollTop)
 * // ... prepend older messages ...
 * const newScrollTop = restoreScrollAnchor(anchor, messages, (m) => m.id, cache, 72)
 * ```
 */
export declare const captureScrollAnchor: <T>(messages: T[], getMessageId: (_message: T) => string, heightCache: ChatHeightCache, estimatedHeight: number, scrollTop: number) => ScrollAnchor | null;
/**
 * Restore scroll position after a history prepend using a previously captured anchor.
 *
 * Finds the anchor message in the (now larger) message array and calculates
 * the scrollTop value that places it at the same visual offset from the
 * viewport top as when the anchor was captured.
 *
 * @param anchor - The scroll anchor captured before prepend
 * @param messages - Updated message array (with prepended messages)
 * @param getMessageId - Function to extract a unique ID from a message
 * @param heightCache - The reactive height cache instance
 * @param estimatedHeight - Fallback height in pixels for unmeasured messages
 * @returns The scrollTop value to restore the visual position, or 0 if anchor not found
 *
 * @example
 * ```ts
 * const scrollTop = restoreScrollAnchor(anchor, messages, (m) => m.id, cache, 72)
 * viewport.scrollTop = scrollTop
 * ```
 */
export declare const restoreScrollAnchor: <T>(anchor: ScrollAnchor, messages: T[], getMessageId: (_message: T) => string, heightCache: ChatHeightCache, estimatedHeight: number) => number;