I use a 10-button mouse (Logitech M705). Under X11, I was able to configure one of the extra buttons to behave as a second middle-click by modifying files in etc/X11/xorg.conf.d.Under Wayland, this no longer works. My research shows that I need to do something with libinput. I thought this would be a common problem but can't find an answer anywhere.Can someone explain how to remap mouse buttons or explain why it can't be done? 
Configure extra mouse buttons under Wayland
mouse;wayland;libinput