First of all, I'm not 100% certain if this is indeed a bug, so my apologizes if I misunderstood anything.

It seems the paintEvent of QWidget is empty, and only implemented in subclasses. I think it should instead draw a PE_Widget so a plain QWidget can be styled with stylesheets, for example to set background colors.

If there is a case this shouldn't be done, at least the stylesheet documentation should point out you can't style QWidgets, only subclasses of it.

This caused some confusion for me which was answered on Stackoverflow.