gistfile1.txt

const Gtk = imports.gi.Gtk;

Gtk.init(null);

let frame = new Gtk.Frame({
    // expand: false,
    margin: 30
});
frame.set_size_request(200, 100)

frame.get_style_context().add_class('shadow')

frame.connect_after('size-allocate', function (widget, clip) {
    clip.x -= 30; clip.y -= 30;
    clip.width += 60; clip.height += 60;
    frame.set_clip(clip)
})

let css = ".shadow {background-color:red; border: 2px solid blue; box-shadow: 0px 10px 20px 10px red;}"

let provider = new Gtk.CssProvider();
provider.load_from_data(css);
let context = frame.get_style_context();
context.add_provider(provider, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION);

let overlay = new Gtk.Overlay();
let grid = new Gtk.Grid({
    expand: true,
    halign: Gtk.Align.FILL,
    valign: Gtk.Align.FILL,
});
grid.set_size_request(200, 100)
overlay.add(grid);

let mwindow = new Gtk.Window({
    default_width: 800,
    default_height: 600
});
overlay.add_overlay(frame)
mwindow.add(overlay);
mwindow.show_all();
mwindow.connect('destroy', function() { Gtk.main_quit() });

Gtk.main();