Synergy is an application I use to share my keyboard and between a VFIO guest and host system. One thing has bugged me about it for ages but today I found a solution that...
By first exporting your config and then selecting the
use existing configuration section you're able to add a couple of key lines to the synergy config which aren't exposed via the UI.
mousebutton(6) = keystroke(WWWBack)
mousebutton(7) = keystroke(WWWForward)
There's probably a bunch of other stuff in the options section but by adding
keystroke() we're able to finally use the forward / back mouse buttons in a Synergy client!