You have to maintain the position during partial postback. For that we need to use
Sys.WebForms.PageRequestManager
Class . It has two eevents
beginRequest
and
endRequest
. During those event you have to trap the dragged position and then set the position again.
Here, I have done a similar application with gridview, for maintaining scroll position during partial postback.
Maintain GridView Scroll Position and Header Inside Update Panel[
^]
Hope this will help you :-D