What you could do is set the width of the sliding pane within the OnClientLoaded event of the RadSplitter. Here is a sample code to do this:
slidingPane.set_width(splitter.get_width() - slidingZone.get_width())
The size of the sliding zone should be subtracted as the sliding pane starts from it onwards. After that the sliding pane will be with the width of the splitter when expanding.
Please, note that docking the sliding pane in this scenario will break the layout of the splitter as a sliding pane with greater width that the containing pane is not a by default supported scenario.
Hope this helps.
the Telerik team
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