[systemsettings] [Bug 395681] New: libinput pointer kcm has no neutral setting, overrides system defaults

[bugzilla_noreply at kde.org]

https://bugs.kde.org/show_bug.cgi?id=395681

            Bug ID: 395681
           Summary: libinput pointer kcm has no neutral setting, overrides
                    system defaults
           Product: systemsettings
           Version: 5.13.1
          Platform: unspecified
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: kcm_mouse
          Assignee: unassigned-bugs at kde.org
          Reporter: grmat at sub.red
                CC: unassigned-bugs at kde.org
  Target Milestone: ---

The new kcm for libinput mouse handling has a slider to set the libinput
acceleration factor which has 10 stops, and therefore lacks a centered/neutral
position.

It is currently not possible to disable mouse acceleration with libinput via
the UI. Many users prefer a 1:1 mapping, and disable acceleration entirely.

A current workaround is to manually change ~/.config/kcminputrc and setting the
value to 0.0 and never touch the kcm slider again.

I'd suggest to make it 11 stops, so we'll have 5 negative, a neutral and 5
positive settings, or add a convenience switch to disable acceleration entirely
(set flat profile and neutral factor).

Please note that the leftmost setting results in strong deceleration (for the
flat profile and for slow speeds in the adaptive profile as well) and not in
disabling acceleration. The value given to libinput is not the speed or
acceleration factor, but a parameter within [-1,1] that will affect the actual
factor. See libinput docs[1] and especially The flat pointer acceleration
profile[2] for further info. libinput has chosen bad terminology here and that
should not be transferred to a DE's UI settings for usability reasons. You are
not setting the "acceleration" with that slider but a parameter that affects
the acceleration. And for the flat profile, it doesn't even actually affect
acceleration but speed.



IMHO, it should also not override the system-wide settings set in
/etc/X11/xorg.conf.d/, at least not initially on installing/upgrading Plasma.
It's ok to override the settings, if the user tweaks the actual kcm manually.
Two reasons:
 - there are Plasma users which have used libinput on X long before the kcm was
updated
 - there are users which have several DEs installed, either due to pragmatism
(some things work better here or there) or because they like to try new things
at times

Those will most likely set their preferences globally. Now when a user has done
that, then upgrades plasma to 5.13 or usually uses Gnome and wants to try
plasma, he'll be left confused with weird mouse behaviour. That also includes
things like inverse scrolling, middle-click emulation, ... and not only the
acceleration factor (which would be bad enough).



[1]:
https://wayland.freedesktop.org/libinput/doc/latest/pointer-acceleration.html
[2]:
https://wayland.freedesktop.org/libinput/doc/latest/pointer-acceleration.html#ptraccel-profile-flat

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