Unfortunately, this is not reproducible here on Linux or any other operating system with neither QCAD 3.32.3 nor QCAD 3.32.3.1.

We also haven't had any similar reports from other users.

Is there anything special about your system or input device such as a special type mouse or an alternative pointing device, multiple pointing devices, etc.?

So far only tried/tested by going version back (to 3.32)
Nothing has changed recently in either my OS or mouse.
Just bog standard mouse and no multiple pointing devices...
I will try different mouses and report back

an update:
tried different mouse - with the same result (in 3-32.2.1) (in 3.32.3.1)
moving mouse up and down Layer List window changes active layer without having to click anything.

System:
Kernel: 6.1.0-38-amd64 [6.1.147-1]
arch: x86_64 bits: 64
compiler: gcc v: 12.2.0
parameters: BOOT_IMAGE=/boot/vmlinuz-6.1.0-38-amd64 root=UUID=<filter> ro quiet splash
Desktop: KDE Plasma v: 5.27.5 wm: kwin_x11 vt: 7 dm:
SDDM Distro: MX-23.6_KDE_x64 Libretto Jan
12 2025 base: Debian GNU/Linux 12 (bookworm)

You are referring to 3.32.2.1 which is a Qt 6.x based development snapshot release. Did you also test with 3.32.2 (stable release, Qt 5.x based)?

sorry made mess of my versions...
yes I am back to 3.32.2
This is the only one that works OK for me

QCAD Professional
Version:
3.32.2.0 (3.32.2)
Internet: QCAD.org
Build Date: Feb 20 2025
Revision: 6616def
Qt Version: 5.14.2
Architecture: x86_64
Compiler: gcc 5.4.0

OK, what about 3.32.3 (stable release, Qt 5.x based)? Thanks.

tested 3.32.3 (stable release, Qt 5.x based)
and it's OK as well

so it seems it is only Qt6 version that is affected.

Thanks for the clarification.

and.. sorry for the confusion

Just to be sure, we are talking about the same thing:

Does the current layer change (indicated by the pen symbol in the layer list) or does the layer selection change (indicated by the different background color of a selected layer or multiple selected layers)?

It is current layer change - pen symbol follows mouse as I move it up or down.
The only way to "prevent" it is to move mouse sideways in line with selected layer

I cannot reproduce this even with MX Linux 23.6 KDE, so I will archive this as an unexplained oddity.

We have a solution, and an explanation why you couldn't replicate it.
It was something as simple as QCAD config file and the fact that I was running both Qt5 and Qt6 versions on the same PC.
When you tried it (same like I did when I tried it using live USB linux) there was no conflict between old and new config file because it was clean install.
The same happened when I re-installed MX (it also wiped out old config files).
So simply wiping out old Qt5 config files and starting clean with Qt6 solves it.
Thank you for your help

I cannot think of a preference that would cause this behavior and I'm mixing Qt 5 and Qt 6 installations all the time. I will put this down as unexplained oddity.