The result you describe is by design. However, your splitter definition is not correct in case you intend to use the dock functionality of the sliding pane. In short, when the sliding pane docks, it re-sizes its parent pane to its size, so that the whole docked sliding pane is visible. However, in this case, the parent pane cannot be re-sized, because it is the only pane in the splitter, and as a result, the sliding pane cannot be docked. That is why, all you need to do, in order to get the result you describe, is add a second pane to the splitter.
the Telerik team