We have a KB article the covers similar issue: http://www.telerik.com/community/code-library/aspnet-ajax/combobox/attach-the-dropdown-to-the-input-element-while-scrolling-within-a-scrollable-container.aspx

The difference with this case is that this particular combobox must observe the scroll of the body. Perhaps it needs additional class name in order to differentiate it from the rest, but the same principles apply.