title: NSTextView with clear background have phantom text
body: <p>I want to implement a textview with transparent background to show subtitles.</p>
  <p>When I set Window backgroundColor = .clear, disable NSTextView draw background, then I change NSTextView.string as you can see in the following picture, <strong>previous text retain in the background</strong>
  <a href="https://i.stack.imgur.com/DzbMH.png" rel="nofollow noreferrer">phantom text image</a></p>
  <p>How can I clear the background text (Foo1245 in the image background) when change NSTextView.string ?</p>
  <p>POC code is here <a href="https://github.com/yalarmorghulis/SingleView" rel="nofollow noreferrer">POC</a></p>