This is a bug with windows used extended title bar.
Seems like DC was not updated.
Use qtbase\examples\widgets\dialogs\classwizard example to reproduce
Steps to reproduce:
- attach auxiliary monitor;
- extend Desktop on both screens;
- make the auxiliary monitor as main screen;
- run sample on main screen. See attached start.png;
- turn off the auxiliary monitor in preferencies, i.e. show Desktop only
on the initial monitor. See result on the attached screenshot