Thank you for the reply, sorry I was on vacation and had no internet access
I'll try to make the problem more clear:
When you read a book it's no problem if the zoomed area moves and you no longer see the lines you just read. However, I want to read scanned mangas(pdf) and there it can be that some speech bubbles and drawings are cut away and if you move the zoomed area you want to see the whole. This means there is a need for a certain overlap. The lower part of the actual area will be the upper part of the next area.