Comment 18 for bug 27724

This isn't really a mouse driver bug, it's a conceptual issue with the X
server's event processing.

Problem being that the SetPointerMapping request by default changes the core
pointer device. Up to X11R7.4, any core event, no matter who actually generates
it, comes from the core pointer and thus assumes the core pointer's mapping
(which in your case is left-handed).

This problem is fixed in Xorg git, but needs a vastly different input system. It
also requires configuration tools to set the button mapping per device, and not
just for the core pointer.