Rust: FLTK: Generic window, borderless, rounded

Cargo.toml

[package]
name = "generic_window"
version = "0.1.0"
edition = "2021"
[[bin]]
name = "generic_window"
path = "src/generic_window.rs"

[dependencies]
fltk = "1.4.0"

generic_window.rs

#![windows_subsystem = "windows"]

use fltk::{
    app,
    enums::{Color, Event, Key},
    image::RgbImage,
    prelude::{
        WidgetExt, WindowExt, GroupExt,
        SurfaceDevice, WidgetBase},
    window::Window,
    surface::ImageSurface,
    draw,
};


fn main() {
    let rounding_radius: i32 = 30;
    let window_size_x: i32 = 200;
    let window_size_y: i32 = 200;
    let window_shape_image = prep_shape(window_size_x, window_size_y, rounding_radius);

    let app = app::App::default();

    // window
    let mut win = Window::default()
        .with_size(window_size_x, window_size_y)
        .with_label("Test")
        .center_screen();

    win.set_shape(Some(window_shape_image));
    // win.set_border(false); // set_shape already does this

    win.end();
    win.show();

    // quit on escape, unfocus
    win.handle({
        move |_wself, event| match event {
            Event::KeyDown => {
                if app::event_key() == Key::Escape {
                    app.quit();
                    true
                } else {
                    false
                }
            },
            Event::Unfocus => {
               app.quit();
               true
            },
            _ => false
        }
    });

    app.run().unwrap();
}


fn prep_shape(w: i32, h: i32, r: i32) -> RgbImage {
    // rounded rectangle
    let surface = ImageSurface::new(w, h, false);
    ImageSurface::push_current(&surface);

    // transparent
    draw::set_draw_color(Color::Black);
    draw::draw_rectf(-1, -1, w + 2, h + 2);
    // visible
    draw::set_draw_color(Color::White);
    draw::draw_rounded_rectf(0, 0, w, h, r);

    let img = surface.image().unwrap();
    ImageSurface::pop_current();
    return img;
}