title: NSVisualEffectView within window no longer blurry in 10.13
body: <p>An existing app that uses <code>NSVisualEffectView</code> (Blending set to Within Window, Material Dark, State Active) no longer seems to 'blur' the content behind it. The app works fine on 10.12 and 10.11 and so on. However with 10.13 beta it appears as a solid, dark opaque overlay on top of the background it's on. All the views in the hierarchy have Core Animation layers switched ON.</p>