From the "Smooth Scrolling" section of [XI2Proto.txt][1]:
> One unit of scrolling in either direction is considered to be equivalent to
> one button event, e.g. for a unit size of 1.0, -2.0 on an valuator type
> Vertical sends two button press/release events for button 4. Likewise, a
> button press event for button 7 generates an event on the Horizontal
> valuator with a value of +1.0. The server may accumulate deltas of less than
> one unit of scrolling.
From [What's new in XI 2.1 - smooth scrolling][2]:
> The increment defines what delta the driver considers to be one scroll
> event. For an increment of +5, each delta of 5 should be regarded as one
> scroll unit down. For an increment of -3, each delta of 3 should be regarded
> as one scroll unit up (i.e. inverted).
[1]: http://www.x.org/releases/X11R7.7/doc/inputproto/XI2proto.txt
[2]: http://who-t.blogspot.com/2011/09/whats-new-in-xi-21-smooth-scrolling.html
This fixes scrolling with my Microsoft mouse in X11 on Debian 8.1.
XInput2 has a concept of master and slave devices,
where a slave device is the actual physical device,
attached to a master device representing the cursor or keyboard
focus.
See http://who-t.blogspot.co.uk/2009/05/xi2-recipes-part-1.html
Mouse events were being received from both the master and slave
devices, but we are only interested in events from the master device.
Fixes#533
Scroll deltas are calculated in X11 by comparing the current and
previous absolute values for the scroll axis when a scroll motion
event is received. If the user scrolls whilst the cursor is outside
of the window then an incorrect delta is reported when the cursor
re-enters the window.
Fix this by resetting the last-seen axis values whenever the cursor
re-enters the window.
* For the moment we're still using plain core X11 events
for handling keyboard activity, so remove the XInput2 code for that
* Small refactoring of X11 input handling and documentation fixes
