/**
 * Resolves after the browser has finished all pending microtasks, effects
 * and renders — i.e. once the current "work cycle" is fully idle.
 *
 * - Uses `requestIdleCallback` when available (Chromium, Firefox) so the
 *   callback only runs when the main thread has no higher-priority work left.
 * - Falls back to `requestAnimationFrame` on browsers that don't support rIC
 *   (Safari).  A single rAF fires after all queued microtasks drain, which
 *   covers the mates effectScheduler + globalScheduler flush cycle.
 * - In SSR (no window) the promise resolves on the next microtask so
 *   server-side code can `await onceIdle()` without throwing.
 *
 * Accepts an optional `callback` that is invoked just before the promise
 * resolves, making both usage patterns work:
 *
 * @example
 * // await style — pause until idle, then continue
 * await onceIdle();
 * console.log("all renders have settled");
 *
 * // callback style — fire-and-forget
 * onceIdle(() => expensiveWork());
 *
 * // callback + await — run work at the idle point, then continue
 * await onceIdle(() => expensiveWork());
 */
export declare function onceIdle(callback?: () => void): Promise<void>;
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