With the classic render mode this allows the developer to always position the popup according to a given target with a given offset.
With the lightweight render mode the current bounds are returned instead of the offsets set by the developer.

The following example always shows a RadWindow according to its offset element instead of only the first time:
						oWindow = $find("<%=winTest2.ClientID%>");
						oWindow.show();
						if (oWindow.get_offsetElementID())
						{
							offsetElementBounds = $telerik.getBounds($get(oWindow.get_offsetElementID()));
							oWindow.moveTo(offsetElementBounds.x + oWindow.get_left(), offsetElementBounds.y + oWindow.get_top());
						}
This will not work as expected with the Lightweight render mode, because the popup will move additionally with its width and height.