After only some hours of practice, I decided that « natural scrolling » was OK. On OSX, no problem: it’s already natural.

On Ubuntu before 12.04 there was initially the xmodmap quirk (inverting some buttons, thanks Andy C.).

It didn’t last longer because of GTK using a different method in 12.04. The scrolling was then half natural (in Chromium), half reversed (in nautilus or gedit). Awkward and hard to live with, for any respectable geek.

Andy C. found the definitive solution again, which BTW completely avoided the xmodmap quirk. Great! A little « manual » because each person must run xinput to find their own numbers and values, but cool anyway because it does the job.

So, my solution is down there, for my MacbookAir3,2 (late 2010):

xinput set-prop 11 284 -237 -237

Note: the only small problem was to find my trackpad. I was expecting to see the word « trackpad » in the xinput list output, but the only place it showed was on the keyboard line (eg. « Apple Internal Keyboard / Trackpad »), which is obviously wrong:

olive@Maocuntu ~ @ xinput list
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ bcm5974                                   id=11   [slave  pointer  (2)]
⎣ Virtual core keyboard                     id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Power Button                              id=8    [slave  keyboard (3)]
    ↳ Sleep Button                              id=9    [slave  keyboard (3)]
    ↳ Apple Inc. Apple Internal Keyboard / Trackpad id=10   [slave  keyboard (3)]
    ↳ Built-in iSight                           id=12   [slave  keyboard (3)]

In fact, the Apple internal trackpad is simply named bcm5974. It’s a Broadcom trackpad (not a Synaptics one), handled by the bcm driver.

This is on Ubuntu 12.10. I don’t remember how it was named on 12.04.
Perhaps this blog post could help some people find their own trackpad more easily.