Strange resolution issue with external screen only - Virtualbox Manager unusable

[SecretCode]

VirtualBox 6.1.42 is working fine for me on my Kubuntu 20.04 host, except when I attach the external screen I have at the office. It's a relatively old screen and lower resolution, but other apps work fine with it.

When the external screen is attached - whether I start vbox before or afterwards - I get a normal window but only a partial display, somehow repeated to fill the window.

VBox Manager 1.png (36.01 KiB) Viewed 1091 times

Keyboard navigation still works, but the display is unusable. This also affects vms, though they display fine when the external screen is not attached.

vbox manager toolbox.png (86.36 KiB) Viewed 1091 times

vbox manager 2.png (57.33 KiB) Viewed 1091 times

I don't even know if this is a vbox problem but as I say other apps work fine with this setup. Does anyone have any suggestions?

It's difficult to find any matching symptoms either here or on the web at large, because when searching for display issues or display corruption the results are all about not getting VMs to display at all (or not getting external screens to work at all).

I don't have another external screen to test with, nor another computer to try on.

eta:
The resolution of the laptop's built-in screen is 3840x2160 but scaled to 2560x1440 because the dpi is ridiculously high .
The resolution of the external screen is 1920x1080.

[scottgus1]

Weird graphics glitches like this on Linux hosts are usually due to some misconfiguration in the Qt graphics, if I remember correctly. What to reconfigure, however, I don't know.

[SecretCode]

I was afraid of that

Do you know of any particular Qt interfaces or libraries that VIrtualBox uses? Since other Qt / KDE apps don't have this issue, I suspect it affects a specific approach.

[fth0]

FWIW, on a Linux host, VirtualBox uses the Qt framework of the host OS. On Windows and macOS hosts, VirtualBox brings its own Qt framework with it.

[SecretCode]

An update: setting KDE "Global Scale" (in System Settings > Display Configuration) to 100% has resolved the problem. I had it at 150% to improve the UI on high DPI screens, but there are other app-specific approaches.