https://bugs.kde.org/show_bug.cgi?id=520222
Bug ID: 520222
Summary: Mouse cursor renders with FPS of foreground window
Classification: Plasma
Product: kwin
Version First 6.6.4
Reported In:
Platform: Debian testing
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: wayland-generic
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Created attachment 192332
--> https://bugs.kde.org/attachment.cgi?id=192332&action=edit
Side by side demonstration with 144 fps on the left and 30 on the right
DESCRIPTION
After updating to Plasma 6.6.4 (from 6.5.3) I've noticed that my mouse cursor
seems to "lag" when on an empty desktop or when hovering over some
applications, but was notably smooth when using Firefox.
After some experimentation I've discovered that this cursor lag only happens
when the cursor is hovering over some window that renders at lower FPS than my
display refresh rate.
My display's refresh rate is 144Hz and it's very jarring when the cursor starts
rendering at 60 FPS.
I have no VRR enabled, in fact it's disabled through the display's OSD.
Enabling it doesn't seem to change the cursor's behavior.
Using KWIN_FORCE_SW_CURSOR=1 solves this cursor lag but adds a different type
of lag when the cursor stutters when I minimize/maximize a window.
STEPS TO REPRODUCE
1. Use a display with high refresh rate
2. Run MANGOHUD_CONFIG=fps_limit=30 mangohud glxgears
3. Observe that while the frame-limited glxgears window is the only one
displaying on the desktop, the cursor starts to visibly skip frames
4. Run MANGOHUD_CONFIG=fps_limit=144 mangohud glxgears (not necessarily closing
the 30 fps one)
5. Observe that the cursor is smooth once more
OBSERVED RESULT
Cursor skips frames when the only foreground window renders at 30 fps.
EXPECTED RESULT
Cursor should still render at 144 fps.
SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux forky/sid
KDE Plasma Version: 6.6.4
KDE Frameworks Version: 6.23.0
Qt Version: 6.10.2
Kernel Version: 7.0.4+deb14-amd64 (64-bit)
Graphics Platform: Wayland
Processors: 24 × AMD Ryzen 9 5900X 12-Core Processor
Memory: 64 GiB of RAM (62.7 GiB usable)
Graphics Processor: NVIDIA GeForce RTX 3090 Ti
ADDITIONAL INFORMATION
Drivers: nvidia-open 595.71.05-1
It's incredibly hard to capture this behavior on video. Trying to record the
screen with OBS and pipewire capture produces a video with a smooth-seeming
cursor. I've tried to record with my phone, but it only supports 60fps video,
and the cursor lag in not obvious.
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