Scrolling increments should be scaled by zoom level and/or window size

[kingjon3377]

Paul writes:

While you're looking at it, could the amount [a scrollbar] increments be scaled
based on current zoom level (so that it takes the same number of clicks
regardless of zoom)? Ideally it would scale based on both zoom level
and window size, but the latter seems less trivial. (And if you want to
go above and beyond, it could detect if the visible area didn't change
and continue incrementing until it does).