// 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);

    panLastX = x;
    panLastY = y;

    // Firefox workaround: get dimensions from parentNode.
    const swidth = svg.clientWidth || svg.parentNode.clientWidth;
    const sheight = svg.clientHeight || svg.parentNode.clientHeight;

    // Convert deltas from screen space to svg space.
    dx *= (svg.viewBox.baseVal.width / swidth);
    dy *= (svg.viewBox.baseVal.height / sheight);

    svg.viewBox.baseVal.x -= dx;