From https://help.gnome.org/users/mousetweaks/stable/mouse-a11y-functions-description.html.en#mouse-a11y-ssc-description "When the user activates this feature, he will be able to perform secondary clicks by using the primary mouse button. This is especially useful for users that have only one mouse button at their disposal." Is this something that's best in libinput or better off in the compositor?
Given the caveats described in the "Differences To The Normal Secondary Click" section of that document, I'd say that this should even be a client feature (i.e. toolkit) which would allow apps to avoid those caveats. That said, and given that this would mean the same work would have to be done for all toolkits and that users of the feature are likely already used to the current behavior (perhaps even because it's the same on other systems?) maybe it is better to do it in the compositor.
I think the compositor is the safer option here as it guarantees the feature is reliable and globally available. just as a note, in another discussion the concept of per-seat a11y features came up, this could be implemented in libinput as a per-seat feature.
closing this buts. This won't be implemented in libinput, see the above comments.
See GNOME bug: https://bugzilla.gnome.org/show_bug.cgi?id=748286
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