RFE: Allow ScrollButton lock

libinput currently supports button scroll, which allows the user to hold a button and use a device's primary motion for scrolling. This is especially useful for trackballs, where the ball is excellent for scrolling.

As a person who suffers from dexterity issues in my hands, I would like to request the ability to use the scroll button to "toggle" the scrolling feature so that I don't have to hold the button down for long periods while scrolling.

Drag lock currently doesn't work for this purpose when applied to the scroll button. From a brief discussion on the mailing list, the technical reason for this is that button scrolling is a libinput feature whereas draglock is a feature implemented in the xorg driver. Thus, by the time drag lock for the scroll button activates, libinput is already done with that button.