Zoom scrolling

Zoom scrolling is a Gtk-only feature, and is one of the reasons that the Gtk developers felt comfortable removing the scrollbar steppers.

Constant-speed zoom scrolling

Long-pressing or shift-clicking on the scrollbar knob, then moving it in a direction, will cause it to move very slowly until you release the mouse click.

Variable-speed zoom scrolling

Right-clicking on the scrollbar outside the knob will cause scrolling at a somewhat faster pace than the constant-speed zoom scrolling. The user can then move the mouse cursor further from the scrollbar to increase the speed.

Further reading

Videos can be found on the Gtk development blog. Note that it uses odd terminology.