no longer set popupNode. It's deprecated.

Author: gkatsev

common/content/buffer.js

@@ -671,7 +671,6 @@ const Buffer = Module("buffer", {
      * @param {Node} elem The context element.
      */
     openContextMenu: function (elem) {
-        document.popupNode = elem;
         let menu = document.getElementById("contentAreaContextMenu");
         menu.showPopup(elem, -1, -1, "context", "bottomleft", "topleft");
     },