Slideshow showing image, then widget with Qt::WA_PaintOnScreen, then QWebEngineView, flicker the first image between each transition

This video shows the attached slideshow-like application running on Windows 10, showing the problem (starts at 00:00:07):

https://www.youtube.com/watch?v=Cpm-XKFvsV4

• Show an widget with an image drawn on paintEvent
• Hide the image widget and show a "video" widget with Qt::WA_PaintOnScreen (the attached example does nothing, but the real world application shows a MediaFoundation video, which Qt's QPainter doesn't know about, and have the same effect)
• Hide the "video" widget, and alternate between 2 QWebEngineView widgets. Between these transitions the image of the first widget appears flickering, most of the times.

I could trace back this to the backing store storing the first widget image on the MainWindow, and then as the "video" widget draw nothing, the first image kept stored on it.

This seems to happen only with widgets that doesn't draw directly using Qt, like QWebEngineView. If the transition is between 2 "image" widgets, this doesn't happen.