When hiding a widget, its allocation is reset to 1x1. But it's realized state remains. Some widgets like the tooltips rely on the fact that calling gtk_widget_realize() would provide an a correct allocation to a widget, but if you've called gtk_widget_hide() on a realized widget, you will never get a correct allocation until the widget is actually shown.

It might be a wrong assumption to call realize() to get a correct allocation. I'm not sure.

Following a quick gjs script showing the inconsistency : 

--------------------------------------------------------
const Mainloop = imports.mainloop;
const Lang = imports.lang;
const Gdk = imports.gi.Gdk;
const Gtk = imports.gi.Gtk;

Gtk.init(null, null);

let w = new Gtk.Window();
w.resize(800, 600);
let box = new Gtk.Box();
w.add(box);
let eb = new Gtk.Button({ label: 'Layout me!' });
box.pack_start(eb, true, true, 0);

let dumpWindowState = function(win) {
    log('window visible : ' + win.visible);
    log('window realized : ' + win.get_realized());
    let alloc = win.get_allocation();
    log('window allocation : ' + alloc.width + 'x' + alloc.height);
    log('-----------');
};

Mainloop.timeout_add_seconds(1, Lang.bind(this, function() {
    dumpWindowState(w);
    w.hide();
    dumpWindowState(w);
    w.realize();
    dumpWindowState(w);
    w.show();

    return false;
}));
                                                                    
Mainloop.timeout_add_seconds(2, Lang.bind(this, function() {
    dumpWindowState(w);
    return false;
}));

dumpWindowState(w);

w.show_all();

dumpWindowState(w);

Gtk.main();