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「Section "InputClass"
Identifier "Orbit Trackball"
MatchProduct "Primax Kensington Eagle Trackball"
Driver "libinput"
Option "ScrollMethod" "button"
Option "ScrollButton" "3"
EndSection」

Thanks to Lenz Grimmer on the #keyboardio IRC channel, I can now scroll with the press of a button and using the ball. Great to validate an idea before building my own device!

#Ommatidia