fitting picture to the display in gtk-rs

Question

Right now I have some simple code that looks like this:

let window = gtk4::ApplicationWindow::new(application);

window.set_default_size(1920, 1080);

let file = <setup for a functional path>;
let picture = gtk4::Picture::for_file(&file);

window.set_child(Some(&picture));
window.show();

The path works and picture renders, so there's no issues there, but there's a small gap of ~10px at the top of the display where the window's border is, despite my display being 1920x1080px. Are there any possible solutions to get the window to fit the entire screen?

I've already tried window.fullscreen() and that causes the image to appear zoomed in (because the raw image I am using is 1440p). Any help is appreciated.