There is a strange bug specific to OS X for Qt 5.11.2 (the bug does not exist in the older Qt 5.7.1 and does not occur on Windows or Linux for Qt 5.11.2).
1. A QGraphicsPixmapItem becomes distorted (see screenshot 3) when calling QGraphicsPixmapItem::setOpacity() with opacity between 0.5 and 1.0. No distortion is noticeable when the opacity is <= 0.5.
2. QGraphicsPixmapItem::setVisible(false) does not make the QGraphicsPixmapItem completely invisible. It is still partially visible until the underlying scene is updated a few more times.
I am using OS X 10.12.6. This issue does not occur on Linux or Windows, just OS X.
I have attached 3 screenshots:
Screenshot 1. (as expected) a pixmap with opacity set to 0.5 using Qt 5.11.2
Screenshot 2. (as expected) a pixmap with opacity set to 1.0 using Qt 5.7.1
Screenshot 3. (unexpected) a pixmap with opacity set to 1.0 using Qt 5.11.2. The image is distorted and and smaller pixmaps which were set to invisible can still be seen (note images).
The pixmap is created with the following code: