problem with validatorcalloutextender position when scroll

Hello,

I used validatorcalloutextender for Error message.

Now i used One panel in modalpopup, and panel is scroll vertically, now when i click on button at the bottom of the panel (scroll down), validatorcalloutextender appers upper position (higher) than textbox is,

and when i click on somewhere else and than in textbox then it reposition fine.

How to solve this problem?

Thanks