KWin Fractional Scaling: mpv resolution 1px too large, renders wrong

mpv Information

mpv v0.41.0 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
 built on Jan  1 1980 00:00:00
libplacebo version: v7.351.0
FFmpeg version: 8.0.1
FFmpeg library versions:
   libavcodec      62.11.100
   libavdevice     62.1.100
   libavfilter     11.4.100
   libavformat     62.3.100
   libavutil       60.8.100
   libswresample   6.1.100
   libswscale      9.1.100

Other Information

- Linux version: NixOS 26.05 (Yarara)
- Kernel Version: 6.12.70
- GPU Model: Radeon RX 5500/5500M / Pro 5500M
- Mesa/GPU Driver Version: Mesa 26.0.0
- Window Manager and Version: KWin Wayland Plasma 6.5.5
- Source of mpv: nixpkgs
- Latest known working version: Unknown
- Issue started after the following happened: Fractional scaling adjusted in KDE Plasma

Reproduction Steps

Set fractional scaling to 185% on my 3072 x 1920 laptop display.

Expected Behavior

Video plays back identically to without fractional scaling.

Actual Behavior

Info reads resolution of 3073 horizontally, and a a tiny slither of the video is cut off (since my display is not 3073 px wide) and the video is not rendered to the pixel grid as it would without scaling.

I can confirm screenshots are correct to what I see on the screen.

Other Tests

I also tested at 190% scaling factor, which should also be fractional scaling, and the resolution of the video reads correctly in mpv info as 3072 across, and it looks pixel perfect compared to using integer scaling.

I also tested Krita (although it does use XWayland) at 185% to see if it would display a canvas the size of my screen at 100% zoom correctly, and it did, so it does seem to be an mpv problem. However, I do wonder if it is related to https://bugs.kde.org/show_bug.cgi?id=505810 or https://bugs.kde.org/show_bug.cgi?id=480171

Log File

Could not play the video with --gpu-debug.

output.txt

Sample Files

No response

I carefully read all instruction and confirm that I did the following:

• I tested with the latest mpv version to validate that the issue is not already fixed.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Reproduce the issue with mpv on a 3072×1920 display at 185% and 190% fractional scaling, then read output.txt from the matching --gpu-debug attempt. Compare the resolution reported by mpv and the rendered edge against the expected 3072-pixel width; done means 185% scaling renders on the pixel grid without cropping or reporting 3073 pixels.