résolu, en collant ça dans xorg.conf :
Section "InputDevice"
Identifier "Synaptics Touchpad"
Driver "synaptics"
Option "SendCoreEvents" "true"
# Option "Device" "/dev/input/mice"
Option "Device" "/dev/input/appletouch"
Option "Protocol" "auto-dev"
Option "LeftEdge" "50"
Option "RightEdge" "900" # max 950
Option "TopEdge" "30"
Option "BottomEdge" "304" # max 334
Option "FingerLow" "15"
Option "FingerHigh" "20"
Option "MaxTapTime" "180"
Option "MaxTapMove" "100"
Option "MaxDoubleTapTime" "180"
Option "ClickTime" "100"
Option "FastTaps" "0"
Option "VertScrollDelta" "30"
Option "HorizScrollDelta" "30"
Option "EdgeMotionMinZ" "30"
Option "EdgeMotionMaxZ" "160"
Option "EdgeMotionMinSpeed" "1"
Option "EdgeMotionMaxSpeed" "400"
Option "EdgeMotionUseAlways" "false"
Option "MinSpeed" "0.5"
Option "MaxSpeed" "2.0"
Option "AccelFactor" "0.12"
Option "LockedDrags" "false"
Option "RTCornerButton" "0"
Option "RBCornerButton" "0"
Option "LTCornerButton" "0"
Option "LBCornerButton" "0"
Option "TapButton1" "1"
Option "TapButton2" "2"
Option "TapButton3" "3"
Option "CircularScrolling" "1"
Option "CircScrollDelta" "0.1"
Option "CircScrollTrigger" "1"
Option "PalmDetect" "false" # not supported
Option "CoastingSpeed" "1"
Option "SHMConfig" "on"
EndSection
