1 rows gets messed up

First off, this is a great piece of coding.
I'm trying the basic-example, full screen with about 200 rows.
My problem is that when I start scrolling (either up or down, by mousewheel or by bar) row 5 or 6 from the top gets messed up, sorta like flips over the other..
As soon as I moved the cursor over it it looks ok again.

It doesn't do it in a like 800x600 window, only when maximized.

Any clues, coz I'm stumped here :-)