Touchpad acceleration

[Stefanos A.]

Hello,

I recently started testing Gnome 3.16 over Wayland 1.7.0 & libinput
0.13.0 and came across the following interesting behavior on my
touchpad. In short, the pointer feels as if it lagging on the monitor
compared to my finger movement. The result is that I consistently
overshoot small targets, such as the "X" button to close a window.

Now, I understand that there is a familiarity effect going on here,
but I have to say that I am not seeing this behavior when using Xorg
or Windows on the same hardware (or OS X on different hardware.)

Moving the "touchpad acceleration" slider to the "slowest" setting
appears to remove this lag, at the cost of a too-slow-to-be-usable
pointer. Moving the slider to the "fastest" setting appears to make
things worse, in the sense that I find it impossible to hit any screen
target reliably.

My theory is that the libinput acceleration curve starts with a <1.0
acceleration for slow movements and moves to a >1.0 acceleration for
faster movements and the lag is a placebo caused by the <1.0 part of
the curve.

Is there a way (configuration or tool) I could use to test this
theory? My gut feeling is that I would prefer to remove or make the
<1.0 part of the acceleration curve less aggressive and keep the >1.0
part, if that makes sense.

`cat /proc/bus/input/devices` returns the following:

I: Bus=0011 Vendor=0002 Product=0007 Version=01b1
N: Name="SynPS/2 Synaptics TouchPad"
P: Phys=isa0060/serio1/input0
S: Sysfs=/devices/platform/i8042/serio1/input/input9
U: Uniq=
H: Handlers=event12 mouse0
B: PROP=9
B: EV=b
B: KEY=6420 30000 0 0 0 0
B: ABS=260800011000003