SysTray-X/webext/js/options_treetable.js

var tree = document.querySelectorAll("ul.tree a:not(:last-child)");
for (var i = 0; i < tree.length; i++) {
  tree[i].addEventListener("click", function(e) {
    var parent = e.target.parentElement;
    var classList = parent.classList;
    if (classList.contains("open")) {
      classList.remove("open");
      var opensubs = parent.querySelectorAll(":scope .open");
      for (var i = 0; i < opensubs.length; i++) {
        opensubs[i].classList.remove("open");
      }
    } else {
      classList.add("open");
    }
    e.preventDefault();
  });
}