QWebView with transparent background does not work properly in Qt 5

In one of our projects, we need a full-screen QWebView with transparent background and content with varying levels of transparency.

The attached example program creates one, seemingly correct at first sight,
but when e.g. arranging desktop windows, on each redraw the yellow semi-transparent part gradually loses transparency. This is a regression from Qt 4.8. On Qt 4.8 the transparency remains consistent even when the QWebView is redrawn.