The Filtering context menu is not positioned properly when there is not enough height. It is positioned depending on the top border of the window, making the filters not accessible.

Screencast: https://www.screencast.com/t/4dQEAczF9c2

Steps to reproduce:
1. Open http://demos.telerik.com/aspnet-ajax/grid/examples/functionality/filtering/excel-like-filtering/defaultcs.aspx
2. Resize the browser so the last elements of the Grid are not visible
3. Open the Filtering context menu

Result: It is shown with too big negative top margin