Difference between update modes is only how much he redraws. FulllViewportUpdate redraws everything - and that why lines disappears after one scroll. With others update modes - it redraws only some small part of screen. And this part is without my lines.

This is screenshoot with SmartViewportUpdate update mode after three moves down and three up.
It looks like when I scroll down - it doesn't redraw lower part of lines. It leaves screen as it was, but in places revealed by scroll there's nothing. The same when I move scroll up - it draws empty space where lines should be. When I get to the top - it draws visible part of line, but rest isn't draw.


The same here - with MinimalViewportUpdate update mode after scroll to the bottom of scene and back.