Widget paint artefacts on MdiSubwindow

[oberlus]

Hello,

I have an QMainWindow application whose central widget is a MdiArea displaying MdiSubWindows. Each subwindow contains a widget which repaints itself every second. The widget draws a pixmap by setting the WA_PaintOutsidePaintEvent to true (simple example code attached). The code presented is a simplification of a much larger application in which many such pixmaps appear and it would be expensive to use update (ie paint event) to update only a few pixmaps.

ok.png

if I overlay two of the subwindows the pixmap for the bottom subwindow shows up in the top sub window

nok.png


I'm trying to determine if this is a bug or if my coding is incorrect. Both 4.7.4 and 4.8.1 show the same behaviour.

Any ideas or suggestion would be very appreciated.