It would be great if in a viewport with only an horizontal scrollbar,
the mouseWheelY(the most common one) would scroll too.
It seems to be handled partially in Viewport but after that the scrollbar only take the wheel value of its orientation.
Maybe something like
float increment = fabs(wheelIncrementY) > fabs(wheelIncrementX) ? wheelIncrementY : wheelIncrementX;
in ScrollBar::mouseWheelMove would be better.
or something specific in Viewport.