prs/prs.desktop/Panels/PostCodes/PostCodePanel.xaml

<UserControl x:Class="PRSDesktop.PostCodePanel"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:PRSDesktop"
             xmlns:dynamicGrid="clr-namespace:InABox.DynamicGrid;assembly=InABox.Wpf"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="1000">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="400"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        
        <local:LocalityTree
            x:Name="LocalityTree"/>
        
        <dynamicGrid:DynamicSplitPanel
            Grid.Column="1"
            Margin="5,0,0,0"
            AllowableViews="Combined"
            View="Combined"
            Anchor="Master"
            AnchorWidth="400">
            
            <dynamicGrid:DynamicSplitPanel.Master>
                <local:CountryGrid
                    x:Name="CountryGrid"
                    OnSelectItem="CountryGrid_OnOnSelectItem"/>    
            </dynamicGrid:DynamicSplitPanel.Master>
            
            <dynamicGrid:DynamicSplitPanel.Detail>
                
                <dynamicGrid:DynamicSplitPanel
                    AllowableViews="Combined"
                    View="Combined"
                    Anchor="Master"
                    AnchorWidth="400">
                    
                    <dynamicGrid:DynamicSplitPanel.Master>
                        <local:StateGrid 
                            x:Name="StateGrid"
                            OnSelectItem="StateGrid_OnOnSelectItem"/>
                    </dynamicGrid:DynamicSplitPanel.Master>
                    
                    <dynamicGrid:DynamicSplitPanel.Detail>
                        <local:LocalityGrid
                            x:Name="LocalityGrid"/>
                    </dynamicGrid:DynamicSplitPanel.Detail>
                    
                </dynamicGrid:DynamicSplitPanel>
                
            </dynamicGrid:DynamicSplitPanel.Detail>
            
        </dynamicGrid:DynamicSplitPanel>
    </Grid>
</UserControl>