This should apply to mice too, not just touchpads.

For example, under X I use this:

Example (for my case), to query:

 xinput --list-props "Logitech G400s Optical Gaming Mouse"
Device 'Logitech G400s Optical Gaming Mouse':
...
        libinput Middle Emulation Enabled (288):        1
...

To set:

xinput set-prop "Logitech G400s Optical Gaming Mouse" "libinput Middle Emulation Enabled" 1

Something on Wayland to do that would be useful.

What is Libinput's virtual middle-click button?

On Wayland the Touchpad KCM has an Emulate middle button checkbox. And I just tested, that it works. Do you mean something else?

(In reply to Roman Gilg from comment #2)
> What is Libinput's virtual middle-click button?
> 
> On Wayland the Touchpad KCM has an Emulate middle button checkbox. And I
> just tested, that it works. Do you mean something else?

For mice, middle click emulation means making left and right click simultaneously, to produce middle click. It helps to relieve strain on the fingers a lot (clicking the scroll wheel is really uncomfortable).

I suppose it's not the same setting as one for touchpads.

Yea, I was more interested in the "virtual middle-click button".

The emulation will work with
* https://phabricator.kde.org/D11468 on Wayland and
* https://phabricator.kde.org/D11469 on X.

You're right, my mistake: the "Emulate Middle Button" does indeed already adequately control this. Turning it on gets rid of Libinput's annoying virtual middle button for my touchpad, turning it into an extension of the virtual left-click button like I would expect (so the left-click button is 2/3 of the width, and the right-click button is the remaining 1/3).