I have this issue I notice that I think it has something to do with the Window Container. When I have a regular ComboBox inside a ChildWindow, it does not behave properly if that ChildWindow is placed inside a WindowContainer.
To recreate this issue: I have a test control that has a ComboBox and a Button like this:
<ComboBox VerticalAlignment="Top" Margin="3">
<ComboBoxItem Content="Option 1"/>
<ComboBoxItem Content="Option 2"/>
<ComboBoxItem Content="Option 3"/>
<ComboBoxItem Content="Option 4"/>
<Button Content="Click Me!" Margin="3" Click="Button_Click" />
I placed this UserControl inside three columns inside a grid. One is directly inside the Grid, the other is wrapped around a ChildWindow. And the last one is wrapped inside a ChildWindow inside a WindowContainer.
<local:TestControl Grid.Column="0" />
<xctk:ChildWindow Grid.Column="1" WindowState="Open">
When I run the test, the Buttons in all three works fine. However, the combo box behaves strangely in the last control. I cannot change the combox item selection with my mouse LeftClick. The only way to get the mouse to work with the combo box is to right-click
on the list while it is opened. Then only after that the combo box behaves properly.