Extra tiles are added when zoomed in, and scrolled up?

Whenever I use any set 20x20, 25x25, 30x30, etc. it always adds what seems to be an additional row of tiles. If I scroll downward, offsetting the centering of the screen, it's fine. However when I scroll back up it immediately adds more and goes away afterwards, it's just really annoying and I've been unable to find any sort of answer to this. 

It's a screen rendering bug and has been around a long time. A common solution is to make your page one or two squares larger all around, giving yourself a margin of emptiness. And added benefit to this is you can hide those margin areas with vanilla fog of war and have a handy place to set "off-screen" monsters without needing to resort to the GM layer. Here's a tip on how you can also use that margin to easily simulate invisible monsters.