title: Qt: Clear previous drawings on repaint
body: <p>I have a totally transparent, top-level, sreen-sized widget, which shows some text waterwarks on macOS. I need to update the system time text on one-minutes intervals. When I call the repaint method, the previously drawn text is not erased completely. Please see the attached image and demo project.</p>
  <p>I've seen several posts on stackoverflow about this topic, but none of them works for me.
  Could anyone show the most correct way to handle it with Qt? Thanks!
  <a href="https://i.stack.imgur.com/MrZot.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MrZot.jpg" alt="enter image description here" /></a></p>
  <p><a href="https://www.dropbox.com/s/k6g94iygeifv7v2/untitled.zip?dl=1" rel="nofollow noreferrer">demo</a></p>