Thanks, @jon121, for making the video to explain. Both in your video and in my brief testing, the graphics artifacts (horizontal and vertical lines) seem to start appearing once you’ve scrolled far enough that the top of the comment is out of view. For that problem, I’m marking this bug report “accepted”.
The other problem, the lack of auto scroll, is related to this merged feature request and this open bug report. The same problem arises when you try to drag the comment itself, or a node, past the edge of the window. Unfortunately it won’t be easy to fix, but it’s covered by the other bug report.