I suppose it is not the page that displays scrollbars in your case (the HTML or BODY element), but another element - a DIV for example. If this is the case, you need to find that scrollable parent of the RadSlider and apply position to it. For example:
The RadSplider uses the position CSS property to position its HTML elements. That is why, in case it is in a scrollable parent, you have to apply position to that parent in order for the slider to scroll with the rest of the content in it.
the Telerik team