evdev interface question: ABS_X without ABS_Y?

I have a quick question about the /dev/input/eventX interface with userspace.

Let's say I have a touchscreen, and I move my finger to (200,100).  I'll see something line this (simplified):

  EV_ABS ABS_X 200
  EV_ABS ABS_Y 100
  EV_SYN

Next, let's say I see something like this:

  EV_ABS ABS_Y 90
  EV_SYN

What is the meaning of that, from the perspective of userspace?  I see two possibilities.  Either:

1. It's a move to (200,90), or
2. It's a move to (0,90)

Which is the correct interpretation?  Is this documented anywhere?

Apparently, the usbhid driver is generating events that imply #1, but xserver-xorg-input-evdev interprets these according to #2.  I'm trying to figure out what needs to be patched.  :)

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Dwayne Litzenberger
PatientWay | Access Management Platform
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