QPainter::drawPixmap produces wrong output in some circumstances

A widget based application displays an image with a zoom factor (for instance 4x) and the GUI handles rubber band selection of pixel aligned rectangle regions in the image.

When switching from Qt 5.4.2 to 5.6.1, the widget repaint when using rubber band selection is totally corrupted.

In the small example reproducing the issue, it seems the bug occurs only when both following conditions are met:

• The QImage has been created from a user supplied memory buffer
• The paint event is triggered by calling update() with a complex clip region containing at least two rectangles