See the upstream bug report -- basically, it sanitises the mouse code so
ZAxisMappings do the right thing, without having to xmodmap your pointer to
enable extra buttons beyond the ZAxisMapping.
Created attachment 1352 [details] [review]
patch from Debian/Ubuntu, very well tested
Since I made ZAxisMapping "4 5" on by default in 6.9, should probably include
this as well.
It looks like the fixes proposed in bug #4915 will fix this in a more general
and configurable way, so I'm going to close this bug as a duplicate of that one.
*** This bug has been marked as a duplicate of 4915 ***