Created attachment 174387 [details]
show artifact

SUMMARY
fractional scaling causes vertical line artifacts when window is positioned via shortcuts

STEPS TO REPRODUCE
1. open app No. 1 (I tested with Konsole, Kate,Keepassxc, bottles, flatseal, dolphin, shotcut, spotify, firefox)
2. Focus app
3. Press Meta+Right arrow key (it only works for the right side, left side doesn't have the artifact) to move window to screen edge (new window tiling/snapping from 5.27 also doesn't have the artifact)
4. open app No. 2 (it spawns so that it overlaps with the previously spawned window)
5. close app No. 2

OBSERVED RESULT
a vertical grey line appears on app No. 1 outlining where app QT app No. 2 used to be
this doesn't happen when I set this monitor to 100% scaling and on my other monitor with also 00% scaling

EXPECTED RESULT
no artifact

SOFTWARE/OS VERSIONS
Operating System: Bazzite 40
KDE Plasma Version: 6.1.5
KDE Frameworks Version: 6.6.0
Qt Version: 6.7.2
Kernel Version: 6.9.12-205.fsync.fc40.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 24 × AMD Ryzen 9 7900X 12-Core Processor
Memory: 30.9 GiB of RAM
Graphics Processor: AMD Radeon RX 6750 XT 

ADDITIONAL INFORMATION
125% scaling 2560x1440, Adaptive Sync on/off doesn't matter

If you drag the app No. 1 to the edges of your display with your mouse it also does that.
This is the old window corner snapping thingy. This is NOT the Plasma 5.27 Tiling/Snapping function.

The artifact seems to disappear when when the Desktop is updates (clock widget on desktop, pressing meta key for kickoff) and when the window is moved

I also encountered this artifact when opening systemsettings > desktop effects and clicking configure and then closing the configure window

App No. 1 can also be GTK apps like flatseal and bottles although its much harder to notice and unlikely to happen in the first place.
When it does, it usually affects the window decoration/title bar (they both use CSDs).

App No. 2 can also be something like spotify and firefox (flatpak, with wayland flags, uses SSDs)

It also appears when you first open konsole (no window snapping of any sort involved) and then open a xwayland app on top of it and then resize it with meta+right click. Seems to be xwayland related as glxgears and lokalize flatpak (also xwayland) have this issue.