Hi Christoph,
Did you set the ToolWindowSingleTabLayoutBehavior property as we described above? When we use this XAML with Coded UI Test, we are able to access all the TextBox controls:
<docking:DockSite ToolWindowsSingleTabLayoutBehavior="Show"><docking:SplitContainer><docking:ToolWindowContainer AutomationProperties.AutomationId="LeftToolWindowContainer"><docking:ToolWindow x:Name="leftToolWindow1" Title="Left 1"><TextBox x:Name="leftTextBox1" /></docking:ToolWindow><docking:ToolWindow x:Name="leftToolWindow2" Title="Left 2"><TextBox x:Name="leftTextBox2" /></docking:ToolWindow></docking:ToolWindowContainer><docking:Workspace><docking:TabbedMdiHost><docking:TabbedMdiContainer AutomationProperties.AutomationId="TabbedMdiContainer1"><docking:DocumentWindow x:Name="document1" Title="Document1"><TextBox x:Name="documentTextBox" /></docking:DocumentWindow></docking:TabbedMdiContainer></docking:TabbedMdiHost></docking:Workspace><docking:ToolWindowContainer AutomationProperties.AutomationId="RightToolWindowContainer"><docking:ToolWindow x:Name="rightToolWindow" Title="Right"><TextBox x:Name="rightTextBox" /></docking:ToolWindow></docking:ToolWindowContainer></docking:SplitContainer></docking:DockSite>
As long as your setup is similar, you should be able to find the child controls ok. Without that ToolWindowSingleTabLayoutBehavior property set, we do alter the UIA Children but Coded UI Test still doesn't seem to find the children if no tabs show. That's why as a workaround, adding that property setting helps when there is a single tool window in a tool window container.