Primary menu has just one entry
Generally speaking menus should have 3 or more entries, with one or two there are usually better patterns (such as just having the buttons be in the toolbar). On top of this there's a GTK4 but that makes menus with just one item too tall, which looks very glitchy:
I'd recommend either adding two more menu entries (e.g. keyboard shortcuts and help?), or dropping the primary menu and just having a button with an "info" icon that opens the about dialog, like e.g. Nostalgia does.