/**
 * Hydration scheduling strategy.
 *
 * - `"visible"` — hydrate when the island first intersects the viewport
 *   (IntersectionObserver, threshold 0). Cheapest deferral for content the
 *   user has not scrolled to yet.
 * - `"idle"` — hydrate during the next idle callback. Falls back to a
 *   `setTimeout(0)` on platforms without `requestIdleCallback`.
 * - `"load"` — hydrate immediately and synchronously. Default when
 *   `when` is omitted.
 * - `"media"` — hydrate when a CSS media query first matches (matchMedia).
 *   Requires the companion `media` prop on `<Island>`. Degrades to immediate
 *   hydration when `window.matchMedia` is unavailable.
 */
export type When = "visible" | "idle" | "load" | "media";
/** Public set of allowed `When` values, useful for runtime validation. */
export declare const WHEN_VALUES: readonly When[];
/** Default `when` strategy applied when `when` is omitted. */
export declare const DEFAULT_WHEN: When;
/** Type guard for `When`. */
export declare function isWhen(value: unknown): value is When;
/**
 * Resolve the validated `when` value, warning in development for unknown
 * inputs. Returns the input unchanged when valid, or [`DEFAULT_WHEN`]
 * otherwise. Lives here (rather than in `island.ts`) so the runtime
 * scheduler can import it without transitively pulling in the JSX
 * wrapper.
 */
export declare function resolveWhen(when: unknown): When;