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 ***
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