I cannot center a widget using GBin.alignment

(* ocamlopt.opt -o squava -w s -I +lablgtk2 lablgtk.cmxa squava.ml *)
(* ./squava *)

let window =
    GMain.init ();
    let wnd = GWindow.window ~resizable:true ~title:"Squava board game" ()
    in  wnd#connect#destroy GMain.quit; wnd

let vbox1 =
    GPack.vbox ~packing:window#add ()

let button id tooltip =
    let but = GButton.tool_button ~stock:id () in
    but#misc#set_tooltip_text tooltip;
    but

let start =
    button `NEW "Start a new game"

let about =
    button `ABOUT "About Squava"

let toolbar =
    let bar = GButton.toolbar
        ~orientation:`HORIZONTAL ~style:`ICONS ~packing:vbox1#pack ()
    in
    bar#set_icon_size `LARGE_TOOLBAR;
    bar#insert start;
    bar#insert (GButton.separator_tool_item ());
    bar#insert about;
    bar

let center =
    GBin.alignment ~xalign:0.5 ~yalign:0.5 ~packing:vbox1#add ()

let asize =
    245
let area =
    GMisc.drawing_area ~width:asize ~height:asize ~packing:center#add ()
let drawing =
    new GDraw.drawable (area#misc#realize (); area#misc#window)

let statusbar =
    GMisc.statusbar ~has_resize_grip:true ~packing:vbox1#pack ()

let dialog = GWindow.about_dialog
    ~authors:
    ["Game design by Cameron Browne & N\xC3\xA9stor Romeral Andr\xC3\xA9s";
    "Program by SpiceGuid (<[email protected]>)"]
    ~copyright:"Copyright \xC2\xA9 2026 SpiceGuid"
    ~license:"GNU General Public License v3"
    ~version:"0.1"
    ~website:"https://boardgamegeek.com/boardgame/112745/squava"
    ~website_label:"Board Game Geek : Squava"
    ~parent:window
    ~destroy_with_parent:true ()

let sqr_size =
    49
let exposed _ =
    drawing#set_foreground `WHITE;
    drawing#rectangle ~x:0 ~y:0 ~width:asize ~height:asize ~filled:true ();
    drawing#set_foreground `BLACK;
    let y = ref 0 in
    while !y <= asize do
        drawing#line ~x:0 ~y:!y ~x:asize ~y:!y;
        y := !y + sqr_size;
    done;
    let x = ref 0 in
    while !x <= asize do
        drawing#line ~x:!x ~y:0 ~x:!x ~y:asize;
        x := !x + sqr_size;
    done;
    false

let () =
    about#connect#clicked (fun () -> dialog#run (); dialog#misc#hide ());
    area#event#connect#expose ~callback:exposed;
    window#show ();
    GMain.main ()