please provide the output of:
qdbus org.kde.KWin /KWin supportInformation

when the problem is visible.

Created attachment 113008 [details]
qdbus command output

Yay (I don't even know if I'm being sarcastic), it just happened again...
When I rebooted before, I ran your command, outputting to a txt file as a baseline.
Now that it happened again, I ran it again and ran a diff on the outputs but sadly no changes.

This time I was working in kicad and when I click on the Start Menu (whatever it's called in KDE), I could see the kicad rendering overlaying the Start Menu.
The text "Project name"..."gerbview"..."pcb_calculator"... all very clear that the rendering was grabbed from kicad.

I might keep it as-is (no reboot) for a while since I can somewhat manage through it, just so we can do more troubleshooting in this current condition rather than have to wait for it to happen.

You are using xrender. I doubt that you want that. Please try switching to OpenGL. If it doesn't work it might indicate broken drivers.

Heh, actually I do want xrender...
The reason I did that (and I probably should've created a bug for it) is because over time, OpenGL rendering messes up video.
I may be watching video on Kodi and rendering "sort of" stops. I say "sort of" because if I move my mouse over it I think, or move the window, the video is repainted, but once I stop moving it, rendering  stops again.
With that, at least I wouldn't have to reboot to "fix" it, but switch the compositor is enough to get it going again (until next time).
XRender NEVER had that problem.

Oh, also I could log out rather than reboot with this issue.
And switching the compositor to OpenGL (without logging out or rebooting) didn't change anything (like how video rendering would start again).

Please switch to OpenGL and provide the supportInformation

Created attachment 113012 [details]
baseline qdbus for opengl

I'm assuming you want it for the corrupted output, but sadly I was forced to do a magic-key-reboot because of a different issue (not related to KDE I don't believe).
But this is the output for opengl after the reboot.
If(/when?) it happens again or messes up with my video viewing, I'll run it again and submit the output.

What I wanted to know was whether your driver works at all. That seems to be the case.

Ah. Do you still think I'm in the right place or should I be looking to some other component as the cause of this issue?
I use Arch, and I think I could pacman -U previous KDE/QT stuff, but I dread that it won't be smooth (dependencies...).

We recently got several graphics issues and what they have in common is the newer Mesa version. If you are able to downgrade Mesa, that could be worth a try.

Given the screenshot I doubt it's a problem in KWin, but rather lower in the stack. What's extremely showing is that xrender hasn't seen any functionality changes for quite some time, so a regression there is extremely unlikely.

Downgraded Mesa to 17.3.7 (from 18.0.4) and seeing it here too.
I think I'll "try to" downgrade mesa and kwin/plasma down to 5.12.1 timeframe.

After downgrading all 5.12.# packages to 5.12.1, I tried to update mesa to 18.0.4 but for some reason, programs wouldn't load, so I brought that down to 18.0.1.

No corruption occurred for a couple of days which I think was good enough, so I updated all the 5.12.1 packages to 5.12.2 and got a corruption today.

I dropped back down to 5.12.1 and will let that go for awhile to see if it truly doesn't corrupt.

Nope, still an issue with 5.12.1.
Dropping mesa down from 18.0.1 to 17.3.

Nope, all kde stuff version 5.12.1, mesa 17.3.5, and the display still corrupts.
Going to drop xorg stuff from 1.20.0-5 to 1.19.5-1.

Man, this is bonkers.

Hmm, downgrading showing xorg-server and xf86-video-intel are in conflict. Maybe something in xf86-video-intel...

Got rid of xf86-video-intel and it still happened.
Downgraded xorg stuff to 1.19.5 and it "looks like" it's still happening except only the background and close button of the program previews is corrupted.
It may be a fluke that the Start menu and desktop icons aren't also corrupted.

I had prior to downgrading xorg stuff, I upgraded everything.
Now I'll just downgrade kde/xorg/mesa altogether.

But really, I'm at a loss. Having downgraded each component that I think could have affected the corruption, yet it still happens.

I started to downgrade a bunch of stuff which started to snowball with dependencies and then sddm ended up no longer logging in (likely unfulfilled downgrades).

So I just upgraded everything to the latest yet again (upgraded during testing so I'm VERY doubtful the success was related to the upgrade) and renamed .config so every config had to be recreated.
After a few days and still no problems.

Go figure...
I still have the old .config dirs but don't have much interest in determining which ended up being the problem since it's working again.

So unless you have an idea you want me to look at relating to the .config stuff, I guess this can be considered closed.

Setting to worksforme as of latest comment.

Ugh, too soon I'm afraid.
(also, seeing I have the option to change the status).

No idea why it happened now. But I will say 2 changes I've made was to the workspace theme/colors and adding widgets.

I've changed them back (I think), but the corruption didn't change. I probably have to log out first for it to clear up.

Just a quick note to say I'm seeing the same thing here on Gentoo:

Version
=======
KWin version: 5.12.5
Qt Version: 5.9.4
Qt compile version: 5.9.4
XCB compile version: 1.13


When it happens, restarting plasmashell works and avoid a full reboot:

$ kquitapp5 plasmashell && kstart5 plasmashell

Good to know. I actually just had to use that right now, but for something different and I'm not even sure what bug category to classify it under...

I just paused spotify then turned off my audio receiver (via HDMI).
I know stuff happens on my PC whenever I turn my receiver on/off. EG. Kodi will come to the foreground.

My desktop and system bar all disappeared this time, almost like a crash except there was nothing listed under coredumpctl and journalctl didn't flag anything in red.
The only entries at that time were a bunch of:
*****
Jun 28 16:49:46 computername ksmserver[2873]: UnmapNotify: 26275158
Jun 28 16:49:46 computername ksmserver[2873]: Unknown toplevel for MapNotify
Jun 28 16:49:46 computername ksmserver[2873]: UnmapNotify: 100663376
*****
and
*****
Jun 28 16:58:49 computername kwin_x11[2883]: QXcbConnection: XCB error: 3 (BadWindow), sequence: 49621, resource id: 111152821, major code: 18 (ChangeProperty), minor code: 0
*****
and a bunch of
*****
Jun 28 16:59:11 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper: RRScreenChangeNotify
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Subpixel Order:  0
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Connection:  "Disconnected"
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Rotation:  "Rotate_0"
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Mode:  70
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         CRTC:  64
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Output:  68
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper: RRNotify_OutputChange
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         State (newValue, Deleted):  1
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Property:  EDIDd
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Output:  68
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper: RRNotify_OutputProperty (ignored)
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         State (newValue, Deleted):  1
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Property:  EDID
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper:         Output:  68
Jun 28 16:59:10 computername kscreen_backend_launcher[2865]: kscreen.xcb.helper: RRNotify_OutputProperty (ignored)
Jun 28 16:58:50 computername ksmserver[2873]: Grab Released
*****

Hmm, actually those ksmserver UnmapNotify related stuff seems to be peppered throughout my log. Not just at that time.
The kscreen_backend_launcher stuff "MAY" be from when I restarted plasmashell.
Again, no crash per se... it just disappears.

Still no corruption ever since I deleted my configs AGAIN. I'll wait some more days before I start to customize it again and also wait some days in between customization. I'm thinking widgets will come first.

Some more debugging lines probably need to be added to the code (somewhere) since the system isn't being told of these failures.

Note to self:
Over 1 week and no corruption after resetting stuff.
Added widgets back. Wait a period of time, then change theme.

2 more weeks later and still no issues after adding ONLY widgets.
I upgraded again and now adding on another change by way of Workspace Theme->Look And Feel-> Oxygen.
We'll see if this causes any issues. I think I normally customize it a little more, but will limit this test to only that change.

Actually I changed to Look And Feel to Breeze Dark and still no issues.
Now I'm taking it further by changing the Desktop Theme to Oxygen, and Colors to Zion (Reversed), and Application Style->Widget Style to Oxygen.

Any update?

Sadly (or not), I haven't had any problems after making those changes.
I also reinstalled opengl intel/mesa stuff the other day and still no problems as of yet.

Maybe the issue was unknowingly fixed by some update. As such, I'll close it with "works for me" since I can't speak for kilian.