/**
 * Keep `data-overflowing` in sync with whether a scroll surface's content
 * actually exceeds its viewport. CSS uses the attribute to switch
 * `overflow-y` between `hidden` and `auto` so iOS does not create a
 * rubber-band scroll layer (and a phantom scrollbar) when there are few
 * messages.
 */
/**
 * Observe `el` and toggle `data-overflowing="true"` whenever
 * `scrollHeight > clientHeight`. Returns a disposer.
 */
export declare function syncScrollOverflowAttribute(el: HTMLElement): () => void;