[BZ#739184] Scrolling in central bar should scroll from screen position instead of cursor
Submitted by Kai Willadsen
Currently, if you scroll with the mouse in the central bar on screen, Meld just runs the next-diff/previous-diff action, which goes to the next/previous chunk relative to the text cursor's position in the textview. This often feel weird because you use the middle-bar scroll in different scenarios to the cursor-relative next/prev.
It would probably be nicer to pick the starting chunk (from which to choose what chunk is next/prev) as whatever chunk is closest to the middle of the screen. This can get slightly weird, since we end up guessing some of the time. However, if we combined this with a heuristic along the lines of "whatever chunk the cursor is in, if that chunk is near the middle of the screen, otherwise whatever chunk is actually in the middle of the screen" then we should be okay.
See discussion starting at: https://mail.gnome.org/archives/meld-list/2014-September/msg00001.html