Summary: synaptics 2-slot touchpads give us pointer jumps when the third finger is down. The data we get is garbage, sometimes the two touches swap slots, sometimes we get two x/y positions within the same frame for the same slot, etc.
As a result, three-finger taps are unreliable.
This wasn't the case with synaptics because it merely relied on BTN_TOOL_TRIPLETAP and didn't use the same motion detection we have in libinput.
Author: Peter Hutterer <email@example.com>
Date: Thu Jun 15 14:13:26 2017 +1000
touchpad: ignore the tap motion threshold if fingers > slots