Embed a YouTube or Vimeo video into a webpage and play that video in QWebView. Now, when you hover the mouse over any of the player controls the video gets distorted for a short time and then gets back to normal until you hover the mouse again. See the attached screenshot (qtbug_webkit.png), it is the same frame with and without the issue.
This problem can be reproduced with Fancy Browser example. Either show the attached html page (qtbug.html) and hover over the player controls or use YouTube with following steps: play any video, click "Share", "Embed", "Show more", and play the preview video and hover the mouse.
I have seen the problem on Windows 7. I was also reported that this happens also on MacOS, but I was not able to verify that. Linux/Ubuntu works fine. The video plays correctly if I load directly the embedded link i.e. https://www.youtube.com/embed/<video_id>. YouTube offers the link in <iframe> tag, but the problem was there also with <object> tag and with other workaround attempts.