In some setups moving simple fixed size widgets/controls causes wild flicker. The attached example fails on my system:

Display 1 on the right 24" 4K 3840 x 2160 (150%)
Display 2 on the left 24" 1920 x 1080 (100%)
Both display use their native resolutions and the recommended scaling. 
The main display does not seem to be important, but it seems like right and left position is.
It obviously has to do with screen change, but unlike QTBUG-62971 it does not have to do with the screen change events (but other events). Defaults will follow. It does not seem to affect Mac. The issue is worsened when there is a drag from the right screen to the left and a move is started on the rightmost part of the window title bar. (That likely causes the screen change event to occur later).