I have a XamDockManager with two document tabs, driven by Prism regions, like so:

 <igDock:DocumentContentHost> 
<igDock:SplitPane igDock:XamDockManager.InitialLocation="DockedLeft" HorizontalAlignment="Stretch"> 
<igDock:TabGroupPane x:Name="SelectorPane" prism:RegionManager.RegionName="{x:Static inf:RegionNames.SelectionRegion}"> 
</igDock:TabGroupPane> 
<igDock:TabGroupPane> 
<igDock:ContentPane x:Name="GridPane" AllowClose="False" CloseButtonVisibility="Collapsed" Header="..."> 
<ContentControl prism:RegionManager.RegionName="{x:Static inf:RegionNames.ContentRegion}"/> 
</igDock:ContentPane> 
</igDock:TabGroupPane> 
</igDock:SplitPane> 
</igDock:DocumentContentHost> 

I have my adapter working, everything is functional.

However.

As defined above, both panes are the same size, with a splitter in the center. I don't want that, so I add Width="400" to the first (left-hand) TabGroupPane... and all of a sudden, I have no more splitter. The connection between the panes is completely lost.

How can I specify the width of the left pane without losing the splitter?