When things like that happens, there might be two reasons:

1. Someone activated QPainter::Antialiasing or QPainter::SmoothPixmapTransform. So Qt tries to spread the lines into multiple pixels. It was disabled when I was writing it, so it is highly improbable.

2. More complicated reason (quite possible). Grid system tries to paint with non-cosmetic pen (width > 0) on a QPainter, which has some transformation applied, therefore the line width gets transformed as well, causing such artifacts. I have implemented a special class KisHandlePainterHelper which helps to easily workaround this problem by handling all the transformations for the user. But, afair, this class was written after the grids code, so most probably grids do the same thing in an ad-hoc way, which got broken recently.