Many mice do not have scroll-wheels. Many that do, do not support
horizontal-scroll.
This is a *non-mandatory* feature and the problem is that there is no
agreed standard here.

From the windows
docs<http://msdn.microsoft.com/en-us/library/windows/desktop/ms645617(v=vs.85).aspx>
:

The wheel rotation will be a multiple of *WHEEL_DELTA*, which is set at
120. This is the threshold for action to be taken, and one such action (for
example, scrolling one increment) should occur for each delta.

The delta was set to 120 to allow Microsoft or other vendors to build
finer-resolution wheels (a freely-rotating wheel with no notches) to send
more messages per rotation, but with a smaller value in each message. To
use this feature, you can either add the incoming delta values until
*WHEEL_DELTA* is reached (so for a delta-rotation you get the same
response), or scroll partial lines in response to the more frequent
messages. You can also choose your scroll granularity and accumulate deltas
until it is reached.

I've seen this value set at 120 on other platforms as well (from a brief
search) but I cannot guarantee it.

As a general note: No one is (or should be) doing production-quality
high-end GUIs with highgui.
Highgui is meant for easy visualization of images - especially at the PoC
or algorithm experimentation phase.
We are not re-implementing wxWidgets.

It is possible to mark this feature as experimental and liable to breaking
changes in the future when more platforms are supported.