some panel code example from Bamberg.

panel.js

const widgets = require("widget");
const data = require("self").data;
const panel = require("panel");

var panelScript = "on('message', function(message) {" +
                  "  console.log('receiving ' + message);" +
                  "  }); "

var wid = widgets.Widget({
  id: "some_menu",
  label: "My Widget Label",
  contentURL: data.url("icon.png"),
  contentScriptWhen: "ready",
  panel: panel.Panel({
//    onMessage: function(message) {
//      console.log(message);
//      },
    contentScript: panelScript,
    allow: true,
    width: 170,
    height: 250,
    contentScriptWhen: "ready"
    }),
  onClick : function() {
    console.log("sending the message");
    wid.panel.postMessage("the message");
    wid.panel.show();
    }
  });