From the screen shot that you have sent us we cannot really understand what exactly is your scenario, and how the problem is reproduced. Is the scroll bar the browser scroll bar or is it a scroll bar of an HTML element? Please note, when the dock is dragged it has the highest z-index on the form and its position becomes absolute. Therefore it will go over any other HTML element on the page if it is dragged or dropped over it (for example if there is a div with a scroll bars it will go over them).
Can you please open a new support ticket and send us a runnable sample project (with clear reproduction steps) where we can reproduce your behavior? Also a larger screen shot will be of great help. Once we receive your project we will do our best to provide a working solution to your project.
the Telerik team