Comment 2 for bug 1226931

The volume menuitem's scroll is following the spec at https://wiki.ubuntu.com/Sound#Sound_menu:

> Whenever the volume item is highlighted:
>
> The Left and Right arrow keys should instantly decrease or
> increase the volume, respectively, to the nearest 5 percent step.
>
> The “-” and “+” keys should instantly decrease or increase the volume,
> respectively, to the nearest 5 percent step.
>
> Rolling a mousewheel up or down should increase or decrease the
> volume respectively, 10% per click.

So (IMO) the problem isn't the scroll behavior, but rather that left/right and -/+ keys don't follow the spec, so there's not a good way to step the volume up & down in the smaller grain. We could handle this in IDO by giving idoscalemenuitem key-press-event and key-release-event handlers as is done in the sibling idoplaybackmenuitem