Made of little things.

/** Short notices over the game, in the page's own HTML (the game's UI is not ours to change). */
export function toast(text, { action, timeoutMs = 6000 } = {}) {
  const box = document.getElementById('toasts');
  if (!box) return () => {};
  const item = document.createElement('div');
  item.className = 'toast';
  item.textContent = text;
  if (action) {
    const button = document.createElement('button');
    button.type = 'button';
    button.textContent = action.label;
    button.addEventListener('click', () => { action.run(); close(); });
    item.append(button);
  }
  box.append(item);
  let timer = timeoutMs > 0 ? setTimeout(() => close(), timeoutMs) : null;
  function close() {
    if (timer) clearTimeout(timer);
    timer = null;
    item.remove();
  }
  return close;
}