// Compute position so menu stays visible and near the mouse.
    const x = Math.min(e.clientX - 10, document.body.clientWidth - menu.clientWidth);
    const y = Math.min(e.clientY - 10, document.body.clientHeight - menu.clientHeight);
    menu.style.left = x + 'px';
    menu.style.top = y + 'px';
    // Set menu links to operate on clicked box.
    setHrefParam('action-source', 'f', box.src);