/** Poll visible page content without overlapping requests. Fleet watches use their own monitor. */
export function pollVisible(task: (signal: AbortSignal) => unknown | Promise<unknown>, interval: number, immediate = true): () => void {
    let stopped = false;
    let running = false;
    let timer: ReturnType<typeof setTimeout> | undefined;
    let controller: AbortController | undefined;
    let failures = 0;
    const visible = () => document.visibilityState !== 'hidden';
    const run = async () => {
        if (stopped || running || !visible()) return;
        clearTimeout(timer);
        running = true;
        controller = new AbortController();
        const timeout = setTimeout(() => controller?.abort(), 30_000);
        try { failures = await task(controller.signal) === false ? failures + 1 : 0; }
        catch (error) { if (!controller.signal.aborted) { failures++; console.error('Page refresh failed', error); } }
        finally {
            clearTimeout(timeout);
            controller = undefined;
            running = false;
            if (!stopped && visible()) timer = setTimeout(run, interval * Math.min(8, 2 ** failures));
        }
    };
    const visibility = () => {
        clearTimeout(timer);
        if (!visible()) controller?.abort();
        if (document.visibilityState !== 'hidden') void run();
    };
    document.addEventListener('visibilitychange', visibility);
    if (immediate) void run(); else timer = setTimeout(run, interval);
    return () => { stopped = true; clearTimeout(timer); controller?.abort(); document.removeEventListener('visibilitychange', visibility); };
}