  const element_name = unique_element_name();
        customElements.define(element_name,
            class MockCustomElement extends HTMLElement {
                disconnectedCallback() { reactions.push("disconnected"); }
            });
        const element = document.createElement(element_name);
        t.add_cleanup(() => element.remove());
        document.body.append(element);
        await Promise.resolve();
        reactions = [];
        document.getElementById("section").moveBefore(element, null)