Two-finger width setting on Dell XPS 15 9650

[Peter Hutterer]

On Fri, Jun 16, 2017 at 05:48:31PM +0000, Vahe Oughourlian wrote:
> On Thu, Jun 15, 2017 at 3:23 PM Peter Hutterer <peter.hutterer at who-t.net>
> wrote:
> 
> > On Tue, Jun 13, 2017 at 01:46:12AM +0000, Vahe Oughourlian wrote:
> > > I'm working with a Dell XPS 15 9650 using libinput. Using the Synaptics
> > > driver, a property is available to set the maximum distance two fingers
> > may
> > > be apart before a two-finger scroll can be triggered, called "Synaptics
> > > Two-Finger Width." At the moment, two fingers anywhere on the touchpad
> > will
> > > switch it over to a two-finger scroll.
> > >
> > > I ran across a post from Peter Hutterer (
> > >
> > https://who-t.blogspot.de/2016/04/why-libinput-doesnt-have-lot-of-config.html
> > )
> > > discussing the intention to minimize options in libinput, and there
> > doesn't
> > > seem to be a setting that affects this behavior that I could find. Given
> > > that, is the most appropriate thing to file a bug for this particular
> > > trackpad to have a reasonable default added for this device, or is this a
> > > setting libinput eventually intends to support? Or is this setting
> > already
> > > available?
> >
> > it's not an available configuration, no. But the main question is what
> > you're actually trying to achive? I'm assuming that any finger distance
> > limits are not the issue per se but that there's something else that
> > affects
> > how you can interact.
> >
> > Cheers,
> >    Peter
> >
> >
> There are a couple common cases:
> 
> 1. My left thumb may drift onto the upper left corner of the touchpad (in
> the usual home-row typing position) while my right hand is controlling the
> touchpad. Despite the 70mm-ish distance between the fingers, the next
> cursor motion by the right hand is treated as a two-finger scroll.

https://bugs.freedesktop.org/show_bug.cgi?id=99703
 
> 2. I often use the physical click as part of a drag-and-highlight. While
> there are dead zones at the bottom of the mouse for the virtual buttons, if
> my thumb isn't quite low enough on the right hand, the moment before
> actually clicking the physical button underneath the trackpad will turn
> into a two-finger scroll. This also can occur at the end of the click as
> I'm lifting the thumb, and the next motion is a scroll.

not quite the same but similar to the bug above. I think that's something
we'd have to revisit once we manage to get a hand on 99703
 
> My intention was to set the distance between two fingers such that they'd
> have to be much closer together (within 20mm, for example) to avoid these
> accidental scroll events.

tbh, I don't think that's the right solution and that's also why there's no
config option for it. Any distance-based guessing of intentions should
always be internal to libinput because once we expose the option we'll have
to keep it around forever. And exposing an option promises behaviour.

I think fixing 99703 will remove most of your issues.

> One thing I have also noticed on other trackpads (on other OSes,
> admittedly) is that both fingers seem to have to be in motion to trigger a
> two finger scroll, whereas I seem to be able to trigger a two-finger scroll
> on the Dell's trackpad by moving one finger while two fingers are on the
> trackpad at a time. Perhaps that's the solution here?

yeah, that's been a long-standing feature of the 2fg scroll on linux. not
sure we can/should get rid of that.

Cheers,
   Peter