/** Isolate body siblings and late portals, restoring only the attributes this function changed. */
export function isolateBackground(keep: HTMLElement): () => void {
    const owned = new Map<Element, string | null>();

    const isolate = (element: Element) => {
        if (element === keep || element.contains(keep) || owned.has(element) || element.hasAttribute('inert')) return;
        owned.set(element, element.getAttribute('aria-hidden'));
        element.setAttribute('inert', '');
        element.setAttribute('aria-hidden', 'true');
    };

    Array.from(document.body.children).forEach(isolate);
    const observer = new MutationObserver((records) => {
        for (const record of records) {
            record.addedNodes.forEach((node) => {
                if (node instanceof Element) isolate(node);
            });
        }
    });
    observer.observe(document.body, { childList: true });

    const holdFocus = (event: FocusEvent) => {
        if (event.target instanceof Node && keep.contains(event.target)) return;
        keep.querySelector<HTMLElement>('button, [href], input, [tabindex]:not([tabindex="-1"])')?.focus();
    };
    document.addEventListener('focusin', holdFocus, true);

    return () => {
        observer.disconnect();
        document.removeEventListener('focusin', holdFocus, true);
        owned.forEach((ariaHidden, element) => {
            element.removeAttribute('inert');
            if (ariaHidden === null) element.removeAttribute('aria-hidden');
            else element.setAttribute('aria-hidden', ariaHidden);
        });
    };
}