Created attachment 123456 [details]
Screenshot (KSysGuard)

SUMMARY
After setting 'Global scale' to 2.0x, there is a gap between some maximised windows and the bottom of the screens. It seems to affect all KDE applications and some Qt based applications (e.g. KeePassXC is affected but Telegram Desktop is not). GTK applications look normal.

I've tried changing theme, disabling xrandr configuration, clearing kcache/qmlcache but the issue persisted as long as I have scale = 2.0x.

STEPS TO REPRODUCE
1. In 'Display Configuration', set 'Global scale' to 2.0x.
2. Restart plasma
3. Launch KSysGuard/Konsole/etc, maximise the window

OBSERVED RESULT
A tiny gap between the window and the bottom of the screen

EXPECTED RESULT
The maximised window occupies the whole screen.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.17.1
KDE Frameworks Version: 5.63.0
Qt Version: 5.13.1
Kernel Version: 5.3.7-arch1-1-ARCH
OS Type: 64-bit

ADDITIONAL INFORMATION
GPU: Nvidia GTX980M
Driver: Nvidia proprietary
Compositor scale method: Accurate
Compositor backend: OpenGL 3.1
Display layout:
┌-----------┐┌----------------------┐┌-----------┐
| 1920x1080 ||                      || 1920x1080 |
└-----------┘|       3840x2160      |└-----------┘
             |                      |
             └----------------------┘