retina画質に対応したものが取得できる。
NSImageの幅高さとCGImageの幅高さは異なることがあるので注意。

下準備

NSScreenとCGImageのExtensionを用意

import AppKit
import CoreGraphics

extension NSScreen {
    var displayID: CGDirectDisplayID {
        let key = NSDeviceDescriptionKey(rawValue: "NSScreenNumber")
        return deviceDescription[key] as! CGDirectDisplayID
    }
}

extension CGImage {
    static func background(_ displayID: CGDirectDisplayID,
                           _ windowID: CGWindowID,
                           _ frame: CGRect) -> CGImage? {
        let bounds = CGDisplayBounds(displayID)
        let windowOptions: CGWindowListOption = [
            .optionOnScreenOnly,
            .optionOnScreenBelowWindow
        ]
        let imageOptions: CGWindowImageOption = [
            .bestResolution,
            .boundsIgnoreFraming
        ]
        return CGWindowListCreateImage(bounds, windowOptions, windowID, imageOptions)
    }
}

画像を取得

class CustomWindow: NSWindow {

    func getScreenShotUnderThisWindow() -> NSImage? {
        let windowID = CGWindowID(windowNumber)
        guard let screen = self.screen,
            let cgImage = CGImage.background(screen.displayID, windowID, frame)
            else { return nil }
        return NSImage(cgImage: cgImage, size: frame.size)
    }

}