200% scaling on wayland touchpad feels laggy on X1 Carbon 9th

[David Zollikofer]

I have an X1 carbon 9th gen with a 4k screen where I use 200% scaling. Unfortunately, the touchpad feels laggy and imprecise when trying to hit click targets.

Switching to X11 helps as well as the flat acceleration profile in gnome tweaks.
I feel like this is related to how gnome or libinput upscale mouse movement.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: gnome-shell 42.5-0ubuntu1
ProcVersionSignature: Ubuntu 5.19.0-42.43~22.04.1-generic 5.19.17
Uname: Linux 5.19.0-42-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Fri Jun 2 16:57:16 2023
DisplayManager: gdm3
InstallationDate: Installed on 2023-05-15 (18 days ago)
InstallationMedia: Ubuntu 22.04.2 LTS "Jammy Jellyfish" - Release amd64 (20230223)
RelatedPackageVersions: mutter-common 42.5-0ubuntu1
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)

[Daniel van Vugt (vanvugt)]

Thanks for the bug report. I would usually recommend trying MUTTER_DEBUG_ENABLE_ATOMIC_KMS=0 to fix this but your logs seem to suggest that mode is already active.

Please:

1. Try disabling these extensions in case they're having some effect:

   '<email address hidden>',
   '<email address hidden>',
   '<email address hidden>',
   '<email address hidden>'

2. Try a USB mouse and confirm there is no such lag.

3. Report your findings to libinput at: https://gitlab.freedesktop.org/libinput/libinput/-/issues

[David Zollikofer (davidrzs)]

This is very subjective and I could be wrong. Upon disabling the extensions the lag was still present. With a USB mouse the lag is not present or at least not as noticable.

Interestingly if I switch to 1080p the issue dissapears.

I think that subjectively the default mouse acceleration for 4k with 200% scaling should behave the same as 1080p with 100% scaling which is not the case (though this might be debatable / there might be good reasons why this is not the case).

Will report this upstream.

[Daniel van Vugt (vanvugt)]

I wonder if libinput is incorrectly scaling up the size of the hysteresis dead zone it uses to ignore finger/sensor jitter. It really should only be scaling up the movement velocity. The hysteresis size shouldn't be scaled because it is in touchpad sensor units, not pixels.

[David Zollikofer (davidrzs)]

Created upstream bug: https://gitlab.freedesktop.org/libinput/libinput/-/issues/907

[David Zollikofer (davidrzs)]

I can confirm that using Kubuntu with Wayland as well as X11 session the issue is not present. Hence, I believe this is a Gnome / Mutter issue.

[David Zollikofer (davidrzs)]

Trying to pinpoint where the problem can be I found that the problem is not present in fedora with the exact same set up. I think there is something in Mutter or Gnome that is different in Ubuntu that let's this bug appear.
Using Fedora fixes the issue for me but I thought I'd post it here for future reference.

[Daniel van Vugt (vanvugt)]

What version of libinput/mutter is in the Fedora setup where the bug is not present? Did you check that Fedora is using Wayland too?

[David Zollikofer (davidrzs)]

Yes runs on wayland.
Libinput 1.23.0
Mutter 44.2
It is hard to actually objectively judge this as stutter/lag is very subjective. Though Fedora feels a lot less fluid (i.e. the cursor doesn't seem to lag a few frames).

[Daniel van Vugt (vanvugt)]

Ubuntu 23.04 is roughly equivalent to that Fedora release so please try 23.04:
https://ubuntu.com/download/desktop

Cursor stutter is a compositor bug we will fix in updates soon: bug 2023766

Fluidity of rendering other than the cursor should be better in Ubuntu because only Ubuntu uses triple buffering on the entire screen.

[David Zollikofer (davidrzs)]

Tested the 23.04 Wayland session and can confirm that it is better! Feel free to close the bug as this will be fixed with updates / future releases. Thanks for the professional bug handling.

[Daniel van Vugt (vanvugt)]

Fix Released in unknown package :)