In high dpi mode (i.e Qt::AA_EnableHighDpiScaling ) folding out a QComboBox can result in the animation to be shown at a very wrong position.
Display 1 primary left (100% - recommended)
Display 2 secondary right (150% - recommended)
1. Run code attached.
2. Move the widget to the secondary
3. Expand the drop-down.
That results in the following when animation is displayed.
(When the animation is done the dropdown is shown correctly, but there can sometimes be 40 items which takes some time to render).
(EDIT: There is plenty of space below the combo (though not visible in the picture - so that has nothing to do with the issue).
I haven't tried with 5.9.x, but I don't recalling seeing the issue there, so I suspect it is a regression.