I've noticed a problem with server filtering when the DropDownList is near the bottom of the page. To reproduce the problem, go to http://demos.telerik.com/aspnet-mvc/dropdownlist/serverfiltering , and shrink the browser window so that the bottom of the browser comes up to to the bottom of the box around the products dropdown. (See attached image for example.)
Click the products dropdown and notice the list is expanded above the control, which is good.
Now type "c", followed by "h". The list now moves below the control, off the page, and it can't be seen. I haven't found a way to be able to see the products in the filtered list, because if I click the scrollbar to try to scroll down the control loses focus and selects the current item, and the same happens if I try to use my mouse's scroll wheel. I am able to arrow down through the products, but it is still very awkward to not be able to see the list.
Also, this example is not a huge problem on this page, because the user could scroll the page down before entering the dropdown, and then be able to see all the contents. But in my app several pages have DropDownLists at the very bottom of the page, which makes it impossible to ever see the filtered list.
I think a good fix for this would be that if the DropDownList starts off expanded above the control, to force it to stay displaying above the control while filtering is taking place.