gistfile1.js

const Gtk = imports.gi.Gtk;

Gtk.init(null);
let w = new Gtk.Window({
    default_width: 800,
    default_height: 600
});
let g = new Gtk.Grid();
let frame = new Gtk.Frame({ expand: true });
let label = new Gtk.Label({
    // Uncomment this to improve the behavior, but not fix it entirely; in
    // passing, this shows that max_width_chars is not working properly either
    // max_width_chars: 1,
    wrap: true,
    expand: true,
    label: 'AAAAA AAAAA AAAAA AAAAA AAAA AAAAA AAAAA AAAAA'
});
let frame2 = new Gtk.Frame({ expand: true });

g.add(frame);
g.add(label);
// Add frame2 instead of label, to see the expected behavior of two "expand"
// widgets in a grid: both are the same size
w.add(g);

// The problem can actually be fixed using a Gtk.SizeGroup
// let sg = new Gtk.SizeGroup({ mode: Gtk.SizeGroupMode.HORIZONTAL });
// sg.add_widget(frame);
// sg.add_widget(label);

w.show_all();
w.connect('destroy', Gtk.main_quit);
Gtk.main();

This looks awesomely minimal!!

Reading about GtkSizeGroup now...