[thristian]

Treating "click in the gutter" as "page up" or "page down" is pretty common, but at least on modern Linux desktops (GTK+3) it means "scroll to here".

On very old Unix desktops (classic Athena widgets), left-clicking anywhere on the scrollbar meant "scroll down" - if you clicked near the top, it would scroll down a little, if you clicked near the bottom it would scroll down a lot, and if you held down the mouse button it would keep scrolling at that speed. Right-clicking meant "scroll up". You had to middle-click to actually move the visible bar to a particular point.

The point of putting the scroll buttons together was to make small adjustments easier. Scroll buttons are fairly small targets, and the other end of the scroll bar can be a long way away, so if the other scroll button is at the other end, it can take a while to travel precisely to it since humans can make small, accurate movements and large, inaccurate movements but not large, accurate movements. On the other hand, if the other button is right next door, you can get there quite easily.