Created attachment 181275 [details]
Contains log.txt from journalctl and main.qml that the log references.

SUMMARY
Monitor scaled to 200% positioned to the right fails to show effects.

STEPS TO REPRODUCE
1. Attach a 1080p and 4k monitor to your PC.
2. In Settings > Display & Monitor, move the 4k monitor to the right of your 1080p monitor and set its scaling to 200%. Apply changes. (Either monitor can be Primary, it doesn't affect the result.)
3. Create a couple virtual desktops in Window Management > Virtual Desktops > Add Desktop.
4. Switch virtual desktops with the shortcut Meta-Ctrl-Right/Left.

OBSERVED RESULT
The left (1080p) monitor on the left successfully displays the animation and transitions to the next/previous virtual desktop, but the right monitor (4k) flickers between its most recent draw before performing the transition and some other draw during/after the transition. Sometimes it flickers between the few most recent draws if elements on the screen had changed just before attempting to switch virtual desktops. (The cursor immediately updates as it's moved around the screen if there are "elements" that change the cursor icon). Eventually, after interacting with the screen (re-maximizing the already-maximized window or dragging around a bit on the desktop), the flickering stops; but it persists until such an interaction is done.

EXPECTED RESULT
Both monitors should display the animation to transition to the next/previous virtual desktop and show the most up-to-date elements on screen.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Linux 6.14.6-arch1-1 (64-bit); KDE Plasma 6.3.5
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0

ADDITIONAL INFORMATION
This only happens if the 4k@200% monitor is positioned to the right of the 1080p monitor, even if it's only slightly to the right (e.g. almost directly above, but a few pixels to the right). Any configuration where the 4k monitor isn't to the right of the 1080p one makes this problem disappear and animations run smoothly as expected.

Initially, I thought this affected all KWin animations, but after some testing, it seems to happen with only certain ones. "Toggle Overview" did have the same effect of not actually showing the overview on the right monitor. However, changing the virtual desktop switching animation to "Fade Desktop" stopped the issue; that one works as expected.

I took a quick look into `journalctl` and I found some errors at the exact time I tried those two desktop effects. I'll add the log as an attachment, as well as the `main.qml` file it seems to be complaining about (regardless of whether or not it seems relevant).

Lastly, it seems there was a similar issue in 2023-2024 (https://bugs.kde.org/show_bug.cgi?id=476002), but I don't believe that's the same. I'll attempt the initial proposed workaround (booting my machine with the 4k monitor at 1080p and changing it back to 4k@200% after the fact) and update this bug report if that remedies it. That should hopefully give an idea of whether or not it's related.