querySelector("main");
    let mainRect;
    if(main) mainRect = main.getBoundingClientRect();
    // First display them all, if they're not already visible.
    for(const panel of panels) {
        panel.classList.remove("unpositioned");
    }
    // Measure them all
    for(const panel of panels) {
        const dfn = document.getElementById(panel.getAttribute("data-anno-for"));
        if(!dfn) {
            console.log("Can't find the annotation panel target:", panel);
            continue;
        }
  