Winit 0.20, the state of windowing in Rust, and a request for help

[oniranger]

Hi, I'm a newbie here. When I try the example on Github page:

use winit::{
    event::{Event, WindowEvent},
    event_loop::{ControlFlow, EventLoop},
    window::WindowBuilder,
};

fn main() {
    let event_loop = EventLoop::new();
    let window = WindowBuilder::new().build(&event_loop).unwrap();

    event_loop.run(move |event, _, control_flow| {
        match event {
            Event::WindowEvent {
                event: WindowEvent::CloseRequested,
                window_id,
            } if window_id == window.id() => *control_flow = ControlFlow::Exit,
            _ => *control_flow = ControlFlow::Wait,
        }
    });
}

I got the following window:

Seems this window captured my screen at that moment and display this picture as it's background. Is this a bug or feature?