Created attachment 176963 [details]
Video Trimmer demonstrating the bug. I had to take a video in OBS and then take a cropped screencap to capture it; spectacle did not work for whatever reason

SUMMARY
In apps that use a moving vertical line to show progress (oftentimes media players like Video Trimmer, REAPER, or Signal's built in audio player for sent audio recordings), the moving line gets duplicated as it moves, though only in certain window positions. If you move the window, the effect might stop. Also, in my testing this only happened in 150% fractional scaling. 125%, 175%, and 200% scaling did not have this issue, or at least not as far as I could tell.

STEPS TO REPRODUCE
1. Set KDE Plasma to 150% fractional scaling in the system settings
2. Run a program like Video Trimmer (https://flathub.org/apps/org.gnome.gitlab.YaLTeR.VideoTrimmer) and start playing a video
3. Observe the progress bar at the bottom

OBSERVED RESULT
The line that shows how far the video has progressed gets duplicated in a trail behind it, and moving the mouse over the duplicated area causes it to disappear like an eraser tool in a drawing program.

EXPECTED RESULT
The progress line simply moves as expected.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20241226
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.9.0
Qt Version: 6.8.1
Kernel Version: 6.12.6-1-default (64-bit)
Graphics Platform: Wayland
Processors: 16 × 12th Gen Intel® Core™ i5-1240P
Memory: 31.1 GiB of RAM
Graphics Processor: Mesa Intel® Iris® Xe Graphics
Manufacturer: Framework
Product Name: Laptop (12th Gen Intel Core)
System Version: A4

ADDITIONAL INFORMATION
- This does not occur in GNOME in my testing (via Fedora 41)
- You can use a 1080p resolution screen to test this. Make sure to scale to 150%.
- This happens across both Wayland and XWayland apps - Electron, GTK, and whatever REAPER uses. I have not seen this in Qt.
- In video trimmer, this bug doesn't occur when the window is unfocused