Hi, thanks you for answer.
The scroll area in my application is inside another widget, the main window, and it has an illustrated background. The problem is for main window not the content of the scroll area.
Anyway, I found a workaround, I set the same background for the main window and the scroll bar, so the scroll bar is opaque but looks transparent.
I still wonder why is there this strange behaviour with the scroll bar, but this topic can be considered as solved as I found a way to get around this.
AFAIK, the Qt 4.8 series has no release officially supporting such a recent version of macOS. Therefore you may have surprises. Note that Qt 4 has reached end of life so unless you are locked to that version, you should really consider updating to Qt 5 which is now at it's 5.9.0 release.