Try moving the vertical bar to resize the area.
The resize is done after you drop the additional bar.


Sample code:

<obspl:Splitter runat="server">
    <LeftPanel>
        <Content>
        
            ... Left Panel content here ...
            
        </Content>
    </LeftPanel>
    <RightPanel>
        <Content>
        
            ... Right Panel content here ...
            
        </Content>
    </RightPanel>
</obspl:Splitter>