As described; to reproduce it's AFAIK enough to have a couple page long Java source code open, then do any trivial edit on it that will make reformat valid, then reformat it, then use undo. The view will scroll to a quite arbitrary location in code, possibly dependent on the amount of lines added/removed/reformatted by the formatting. It's possible to "scoll" the view (note: even when in the middle of it!) by constantly using "reformat" and "undo" after each other, as any format-undo sequence will shift the view by some lines.

In most extreme cases (for long files, >=1k lines) the view can even jump by a couple of view pages.

Possibly related (though fixed) https://netbeans.org/bugzilla/show_bug.cgi?id=183998