Tested in X11 where its fairly confusing. buttons 4 and 5 are used for the wheel which is well known, but it seems 6 and 7 are used for horizontal scrolling, my mouse assigns the extra 2 buttons to events 8 & 9. So the X11 events used for buttons called 4&5 in blender are 8&9 in X11. The mouse buttons can be re-ordered like this once xorg starts (swaps 6,7 with 8,9) xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7" Couldn't test Win32, Apple not supported. If someone wants to add horizontal scrolling its quite easy.