One of the recurring questions about to input event handling is how relative and absolute devices are handled in X. Surprisingly, the answer is "that's complicated" and it took me a while to wrap my head around this.
Back in the days of yonder, devices had no mode. X (the coordinate, not the window system) was x, y was usually y unless bugs intervened and we all had mice, computers the size of fridges and fridges the size of, well, fridges. Such was the lore, blah blah (those of us who are old enough to actually remember those days, which excludes myself for example, may remember them fondly).
In 1994, the new kid on the block (not one of the singing ones, they disbanded that same year1, presumably for other reasons. Though that'd be one thing I'd love to be credited with...), the X Input Extension protocol added support for device mode and the notion of relative and absolute devices was born (within the Xiverse, anyway). Fast-forward half a lifetime of something that lives for 30 years and XI2 added to that per-valuator modes. i.e. a device may have an absolute X axis and a relative Y axis.
But: the device mode in X describes the behaviour of a device only. On an absolute device, the pointer is expected to jump to the position the device tells it. On a relative device, the pointer is expected to move by the coordinates provided, based on the current pointer position (and accounting for pointer acceleration).
However, there are no requirements for how to send coordinates and absolute devices may send relative data and vice versa. So inside the server, the mode of a device (or a valuator) has no meaning. It's merely a flag sent to the client.
Which makes mode processing a bit tricky because the only time we know if an event was relative or absolute is when we receive the actual event (events include a absolute/relative flag). An absolute device may calculate deltas based on the previous position and then submit the new position as absolute coordinate - and thus feel like a relative device to the user. For whatever reason you'd want to do this, I don't think any driver does it atm (though I have vague recollections of having done this in the past). But it just adds to the insanity that is input in X.
Either way, we cannot rely on the device mode for anything in the server, and the actual device mode is just a hint to the client.
1I had to wikipedia this. I wonder what that does to my reputation when the admins see the logs.