/** Poll visible page content without overlapping requests. Fleet watches use their own monitor. */
export function pollVisible(task: () => void | Promise<unknown>, interval: number, immediate = true): () => void {
    let stopped = false;
    let running = false;
    let timer: ReturnType<typeof setTimeout> | undefined;
    const visible = () => document.visibilityState !== 'hidden';
    const run = async () => {
        if (stopped || running || !visible()) return;
        clearTimeout(timer);
        running = true;
        try { await task(); }
        catch (error) { console.error('Page refresh failed', error); }
        finally {
            running = false;
            if (!stopped && visible()) timer = setTimeout(run, interval);
        }
    };
    const visibility = () => {
        clearTimeout(timer);
        if (document.visibilityState !== 'hidden') void run();
    };
    document.addEventListener('visibilitychange', visibility);
    if (immediate) void run(); else timer = setTimeout(run, interval);
    return () => { stopped = true; clearTimeout(timer); document.removeEventListener('visibilitychange', visibility); };
}