I have already answered your support ticket on the subject. For convenience I will paste my answer here as well:
If I understand you correctly you need to have the sliding pane expanded only on initial load and collapsed on subsequent postbacks. To achieve this, you need to manually clear the ExpandedPaneId property of the parent sliding zone, e.g.:
sender, EventArgs e)
You need to manually clear the ExpandedPaneId because by design RadSplitter preserves the its state over postbacks.
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