Bug 513277

Summary: With direct scanout enabled, on latest kernel desktop background image bounces up and down a pixel on secondary monitor
Product: [Plasma] kwin Reporter: moichumoigaming
Component: wayland-genericAssignee: KWin default assignee <kwin-bugs-null>
Status: REPORTED ---    
Severity: normal CC: kdedev, nate, richardjacton, xaver.hugl
Priority: NOR Keywords: multiscreen, regression
Version First Reported In: 6.5.0   
Target Milestone: ---   
Platform: Other   
OS: Linux   
Latest Commit: Version Fixed/Implemented In:
Sentry Crash Report:
Attachments: drm_info output when off by 1 pixel
drm_info output when correct

Description moichumoigaming 2025-12-13 01:17:59 UTC 
SUMMARY
My secondary desktop background image, when I move my cursor to it from my primary screen, and when right-clicking the context menu with my cursor on it, bounces up and down a pixel, visible at the top of the screen.

STEPS TO REPRODUCE
1. Have a primary monitor with fractional scaling - I use 150% - and of a higher resolution than your secondary monitor. I use a 3840x2160 monitor.
2. Have a secondary monitor with fractional scaling - I use 150% - and of a lower resolution than your primary monitor. I use a 1920x1080 monitor.
3. Move your cursor between the two screens. Right-click the desktop on the secondary monitor.

OBSERVED RESULT
The desktop background image will bounce up and down a pixel on the secondary monitor. Visible at the top of the screen.

EXPECTED RESULT
The image should remain stable.

SOFTWARE/OS VERSIONS
Operating System: CachyOS Linux
KDE Plasma Version: 6.5.4
KDE Frameworks Version: 6.20.0
Qt Version: 6.10.1
Kernel Version: 6.18.0-3-cachyos (64-bit)
Graphics Platform: Wayland

GPU1 : AMD RX 9070 XT
GPU2 (headless, connected to a display, but not outputting by kwin): NVIDIA GeForce GTX 1080 Ti

My screen setup :
Left : 1080p 60hz, 150% scaling, 10 bit colors.
Middle : 2160p 240hz with DSC (display stream compression), 150% scaling, 10 bit colors.
Right : 1080p 60hz, 100% scaling, 10 bit colors.

ADDITIONAL INFORMATION
This does not happen on my third non scaled 1080p monitor.
This appeared in plasma 6.5.0.

This does not happen on the LTS kernel of my distro, which is kernel 6.12.60-2-cachyos-lts.
Comment 1 Nate Graham 2025-12-16 22:30:04 UTC 
Seems like a kernel regression, and thanks for doing that troubleshooting! Can you report this to the CachyOS folks!
Comment 2 Zamundaaa 2025-12-16 22:33:39 UTC 
First, can you check if it still happens if you set https://invent.kde.org/plasma/kwin/-/wikis/Environment-Variables#kwin_drm_no_direct_scanout to 1?
Comment 3 Zamundaaa 2025-12-16 22:33:49 UTC 
.
Comment 4 moichumoigaming 2025-12-16 23:44:21 UTC 
On the latest kernel, 6.18.1-2-cachyos, with KWIN_DRM_NO_DIRECT_SCANOUT=1, the issue is fixed!
Comment 5 Zamundaaa 2025-12-19 18:55:31 UTC 
Okay, can you remove it again, and then get the drm_info output while the image is mispositioned by that one pixel?
You can do
> sleep 5; drm_info
+ minimized the terminal, or run the command from ssh
Comment 6 moichumoigaming 2025-12-20 16:33:49 UTC 
Created attachment 187818 [details]
drm_info output when off by 1 pixel

Here is the output when the image is mispositioned by one pixel. Let me know if you would like the output when the image is not mispositioned as well. I attached the output in a txt file, because it is too long to leave in a comment.
Comment 7 moichumoigaming 2025-12-20 16:42:07 UTC 
Created attachment 187819 [details]
drm_info output when correct

Just to be proactive, here is the output when the image is correct (not off by 1 pixel).

Running the "diff" command on both text files, this difference may be relevant:
1286c1286
<     │       ├───"CRTC_Y" (atomic): srange [INT32_MIN, INT32_MAX] = 1
---
>     │       ├───"CRTC_Y" (atomic): srange [INT32_MIN, INT32_MAX] = 0

Top line is when off by one pixel, bottom line is when correct.