Created attachment 180836 [details]
The screenshot

The breeze theme for gtk4 application shows a wierd round cornering inside the headbar


STEPS TO REPRODUCE
1. Implement a simple gtk4 app that uses system title bar

OBSERVED RESULT
A round corners below the system headbar, black background on the removed part


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch linux
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0

ADDITIONAL INFORMATION
The rust snippet to reproduce:

let app = gtk::Application::builder().application_id("IIII").build();

app.connect_activate(|app| {
    let window = gtk::ApplicationWindow::builder()
        .title("test")
        .application(app)
        .build();

    window.set_child(Some(&{
        let vbox = gtk::Box::new(gtk::Orientation::Vertical, 8);

        vbox.append(&gtk::Label::new(Some("hello, world")));

        vbox
    }));

    window.present();
});

app.run()