Comment 9 for bug 1637688

The old way of mouse wheel scrolling was mice would emit discrete button 4 and 5 events, and scrolling was jumpy.

The new way of mouse wheel scrolling is via XInput2, you get two additional axes (vertical and horizontal scroll events) that give you more fine-grained positions. This way you can get smooth scrolling with a touchpad.

When you remap mouse buttons, this can only affect older applications that do not support smooth mouse wheel scrolling.