default gnome-shell extension

gistfile1.js

// Sample extension code, makes clicking on the panel show a message
const St = imports.gi.St;
const Mainloop = imports.mainloop;

const Main = imports.ui.main;

function _showHello() {
    let text = new St.Label({ style_class: 'helloworld-label',
                              text: "Hello, world!" });
    let monitor = global.get_primary_monitor();
    global.stage.add_actor(text);
    text.set_position(Math.floor (monitor.width / 2 - text.width / 2),
                      Math.floor(monitor.height / 2 - text.height / 2));
    Mainloop.timeout_add(3000, function () { text.destroy(); });
}

// Put your extension initialization code here
function main() {
    Main.panel.actor.reactive = true;
    Main.panel.actor.connect('button-release-event', _showHello);
}