Comment 5 for bug 139342

and if you still have weirdness so it's the white line that is drawn in the wrong position.
take a look at the code.
These cairo bugs are due to the offset sometimes, for example
  cairo_rectangle (cr, x, y + 0,5, width, height);
while it should be
  cairo_rectangle (cr, x, y + 0,5, width, height-1);