This issue is hardware related. Originally from KDE Bugzilla https://bugs.kde.org/show_bug.cgi?id=385885

Preparing:

1. Have two monitor with different resolutions, place them side by side.
2. In KDE, set HiDPI scale factor to 1.5.
3. Reboot or log off to enable scale.

Test with flameshot or spectacle screenshot tool:

1. Open flameshot or spectacle.
2. Choose Rectangular Region mode.
3. Take a screenshot.

You will see the screen content (actually a full screen widget) has all kinds of wired offset like this:

Test with Qt Creator:

1. Open Qt Creator.
2. Create a Qt Widget application project.
3. Next.

You may see this wired window:

This issue won't appear if you either:

1. Have only one screen.
2. Have two screen with same resolution.
3. Don't scale.