Ext.setup({
    fullscreen: false,
    onReady: function(){
        var butLogin = new Ext.Button({
            text: 'Login',
            ui: 'round',
            handler: function(){
                var msg = new Ext.MessageBox({
                    centered: true
                });
                msg.alert('info', 'ok');
            }
        });

        var tela = new Ext.Panel({
            items: [butLogin],
            layout: {
                type: 'vbox'
            },
            width: 300,
            height: 300
        });
        tela.show();
    }
});