Textfield scroll lock

Hi, guys!

Very strange problem. I have Dynamic textfield attached to the movie and then I put some text inside of it. Every line of the text fits to the textfield size perfectly.

mytext.autosize = true;

but! if I roll my mouse over the textfield and then try to use scrolling wheel I notice that it actually scrolls and the first line disappears. Of course I can scroll back, but why it have reaction on to the scroll wheel if ALL content fits perfectly?

Is there any way to lock scroll?

Thank you in advance