Regression: background of transparent window is not cleared before call to paintEvent()

the background of transparent windows is not cleared to tranparency before paintEvent() is called. I could not find any method to clear it to full transparency before drawing my own semi transparent background. so i have no workaround.
it worked in Qt 4.8.4.
when you start the sample program, you first get a window as in img 1. note: you even get this if you have NOT reimplemented paintEvent() at all! The contents just happens to be nearly what is expected because probably the same buffer is used as for creating the background image from file just before. The widget already contains these pixels when paintEvent() is entered.
if you enlarge the window, (adding transparent pixels only, so don't be too astonished) the background is drawn as desired, see img 2. note the difference to img 1, which is probably due to one of both being premultiplied alpha and the other not.
If you shring the window again you get what is seen in img 3, revealing the not erased background of the window.