I have a sample application which contains a XamlDockManager as the content for a XamRibbonWindow. In that dock manager I have a number of TabGroupPanes and a DocumentContentHost. As I drag a TabGroupPane over the center drop zone of the DocumentContentHost pane, the application locks up with the exception in the subject line. Sometimes it take a couple tries, but I seem to have isolated it pretty well and can easily and consistently reproduce in a few seconds.

It looks like I can drag around all day in other TabGroupPanes, including the center drop zone of each, and I can create a new SplitPane in the DocumentContentHost. I just can't dock it this one way. 

In researching this further, I thought I'd use a workaround and set the AllowInDocumentHost property to false on the TabGroupPanes. When I do that, I don't get the center drop zone highlight, but it seems to catch an edge or something and will still fail.

Not being able to dock TabGroupPanes in the DocumentContentHost is actually preferred in our app. Is there a way to disable it altogether so you don't even see the adorners on the DocumentContentHost?

Here's the markup for the dock manager. We're using version 9.1 with I think the latest service releases.

<igDock:XamDockManager x:Name="dockManager">
<igDock:XamDockManager.Panes>
<igDock:SplitPane x:Name="ExplorerSplit" igDock:XamDockManager.InitialLocation="DockedLeft" Width="225">
<igDock:TabGroupPane x:Name="ExplorerPane">
<igDock:ContentPane x:Name="Explorer" IsPinned="True" Header="Explorer">
<Button Content="Explorer" IsEnabled="False" />
</igDock:ContentPane>
</igDock:TabGroupPane>
</igDock:SplitPane>
<igDock:SplitPane x:Name="ListSplit" SplitterOrientation="Vertical" igDock:XamDockManager.InitialLocation="DockedTop" Height="145">
<igDock:TabGroupPane x:Name="ListPane">
<igDock:ContentPane x:Name="List" IsPinned="True" Header="List">
<Button Content="List" IsEnabled="False" />
</igDock:ContentPane>
</igDock:TabGroupPane>
</igDock:SplitPane>
<igDock:SplitPane x:Name="DetailSplit" SplitterOrientation="Vertical" igDock:XamDockManager.InitialLocation="DockedRight" Width="270">
<igDock:TabGroupPane x:Name="DetailPane">
<igDock:ContentPane x:Name="Detail" IsPinned="True" Header="Detail">
<Button Content="Detail" IsEnabled="False" />
</igDock:ContentPane>
</igDock:TabGroupPane>
</igDock:SplitPane>
</igDock:XamDockManager.Panes>
<igDock:DocumentContentHost x:Name="DocumentHost" Background="DarkGray">
<igDock:SplitPane x:Name="DocumentsSplit" SplitterOrientation="Vertical">
<igDock:TabGroupPane x:Name="DocumentsPane" TabStripPlacement="Bottom">
<igDock:ContentPane x:Name="Document" Header="Document">
<Button Content="Document" IsEnabled="False" />
</igDock:ContentPane>
</igDock:TabGroupPane>
</igDock:SplitPane>
</igDock:DocumentContentHost>
</igDock:XamDockManager>