Textfield and mouse wheel problem

hey all!

ive just made a scrollbar with lots of interaction, it eases the scroling, the scroll thumb resizes depending on the size of the content, when you click on the scrolling track, the scroll thumb moves to the mouseY and scrolls the text to the right place.
The last feature i have added is mouse wheel interaction, this works just how its supposed to and scrolls the text accordingly, but for some reason it also moves the text field up or down by one line of text, i cant figure out why its doing this.

thanks in advanced:D