static JS js_fixedBackgroundImage(S imgURL) {
  if (empty(imgURL)) null;
  ret JS([[
    var target = document.querySelector("body");
    target.style.cssText = $style;
  ]], style := nlToSpace(replaceDollarVars([[
    background-image: url($imgURL);
    background-attachment: fixed;
    background-position: center;
    background-repeat: no-repeat;
    background-size: cover;
  ]], +imgURL)));
}