WPF: Focus in a Window and UserControl - wpf

Question

I'm trying to get a UserControl to tab properly and am baffled. The logical tree looks like this.
|-Window
-Grid
-TabControl
-TabItem
-StackPanel
-MyUserControl
|-StackPanel
-GroupBox
-Grid
-ComboBox
-Textbox1
-Textbox2
Everything works fine, except when the visibility converter for the ComboBox returns Visibility.Collapsed (don't allow user to change database mode), then when textbox1 is selected, instead of being able to tab through the controls in the UserControl, the focus shifts to a button declared at the bottom of the window. Nothing else apart from the controls displayed has TabIndex or FocusManager properties set.
I'm banging my head against a brick wall and I must be missing something. I've tried IsFocusScope=True/False, played with FocusedElement and nothing works if that ComboBox is invisible (Visibility.Collapsed).
<Window x:Class="MyNamespace.Client.WinInstaller"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
FocusManager.FocusedElement="{Binding ElementName=tabWizard}">
<Window.Resources>
<props:Settings x:Key="settings" />
</Window.Resources>
<Grid Grid.IsSharedSizeScope="True">
<!-- row and column definitions omitted -->
<loc:SmallHeader Grid.Row="0" x:Name="headerBranding" HeaderText="Setup" />
<TabControl x:Name="tabWizard" DataContext="{StaticResource settings}" SelectedIndex="0" FocusManager.IsFocusScope="True">
<TabItem x:Name="tbStart" Height="0">
<StackPanel>
<TextBlock Text="Database Mode"/>
<loc:DatabaseSelector x:Name="dbSelector" AllowChangeMode="False" TabIndex="1"
AvailableDatabaseModes="SQLServer" IsPortRequired="False"
DatabaseMode="{Binding Default.DbMode,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
DatabasePath="{Binding Default.DatabasePath,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
</StackPanel>
</TabItem>
...
The top of the user control is below:
<UserControl x:Class="MyNamespace.Client.DatabaseSelector"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Name="root"
FocusManager.IsFocusScope="True"
FocusManager.FocusedElement="{Binding ElementName=cboDbMode}">
<UserControl.Resources>
<conv:DatabaseModeIsFileBased x:Key="DatabaseModeIsFileBased"/>
<BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter"/>
</UserControl.Resources>
<StackPanel DataContext="{Binding}">
<GroupBox>
<Grid>
<!-- row and column definitions omitted -->
<Label Content="Database Mode"/>
<ComboBox x:Name="cboDbMode" SelectedValue="{Binding ElementName=root,Path=DatabaseMode,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
DisplayMemberPath="Value" SelectedValuePath="Key" TabIndex="1" Visibility="{Binding AllowChangeMode,ElementName=root,Converter={StaticResource BooleanToVisibilityConverter}}" />
<!-- AllowChangeMode is a DependencyProperty on the UserControl -->
<Grid><!-- row and column definitions omitted -->
<Label "Host"/>
<TextBox x:Name="txtDBHost" Text="{Binding ElementName=root,Path=DatabaseHost,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" TabIndex="2" />
<TextBox x:Name="txtDBPort" Text="{Binding ElementName=root,Path=DatabasePortString,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" TabIndex="3" />

Answer 1

I know this response is quite late... but have you tried:
<UserControl ... KeyboardNavigation.TabNavigation="Local">
Doing so will ensure once your UserControl has recieved focus, you will navigate only through TabStop within your UserControl (instead of worring about conflicting TabIndex values throughout your app). After looping through the TabStops of your UserControl, TabNavigation will resume to the TabStop outside of it.
http://msdn.microsoft.com/en-us/library/system.windows.input.keyboardnavigationmode.aspx

Answer 2

Maybe the problem is that you hide the FocusManager.FocusedElement.
Anyway, you could make life easier by just eliminating some complicating factors:
Remove FocusManager.FocusedElement...
the ComboBox is the first control within anyway.
Remove FocusManager.IsFocusScope...
I suppose it's ok if everytime you enter the usercontrol the first control within will be focussed, not the one that was focussed when you left it before. Just let the usercontrol be "inlined" in the surrounding controls.
Remove the explicit TabIndices in the UserControl. Your layout already implies the same ordering.
If you eliminate these three complicating factors, you might already be done.
Maybe you also have to set your UserControl Focusable=False, s.t. the focus is passed to the first focussable Control within - comboBox or TextBox1.