Specified Tab Order is not being followed?

Question

Why isn't my XAML following the TabOrder I specified?

I currently have:

<DockPanel>
    <Grid DockPanel.Dock="Top">
        <UserControl TabIndex="0">
            <StackPanel Orientation="Horizontal">
                <ComboBox />
                <TextBox Text="Search Text" />
                <Button Content="Search" />
            </StackPanel>
        </UserControl>
        <ComboBox TabIndex="1" />
        <Separator />
        <TextBox TabIndex="3" Text="Save" />
        <TextBox TabIndex="4" Text="Cancel" />
    </Grid>
    <Grid>
        <ContentControl TabIndex="2" />
        <Popup />
    </Grid>
</DockPanel>

My TabOrder should go

• Search ComboBox
• Search Text
• Search Button
• Database ComboBox
• ContentControl
• Save Button
• Cancel Button

But instead it goes

• Search ComboBox
• Search Text
• Search Button
• ContentControl
• Database ComboBox
• Save Button
• Cancel Button

What do I have wrong with my TabOrder?

Edit

I found this SO answer which suggested making UserControl.IsTabStop="False", and binding it's Child control's TabIndex to UserControl.TabIndex, which partially works.

My TabOrder is now

• Search ComboBox
• Search Text
• Search Button
• Database ComboBox
• Save Button
• Cancel Button
• ContentControl

Answer 1

Apparently by default, WPF reads all the controls, inside and outside UserControls, at the same tab level (unless specified otherwise). Since the controls inside the UserControl do not have a TabIndex specified, they get tabbed to last after the first tab cycle.

The workaround was to bind the TabIndex of the inner controls to the TabIndex of the UserControl

<DockPanel Margin="10" KeyboardNavigation.TabNavigation="Cycle">
    <Grid DockPanel.Dock="Top"
          local:GridProperties.ColumnCount="6"
          local:GridProperties.StarColumns="0">

        <TextBlock Text="Header" FontSize="20" FontWeight="Bold" />
        <ContentControl Grid.Column="1" TabIndex="0" IsTabStop="False" Content="{Binding SearchViewModel}" />

        <ComboBox Grid.Column="2" Margin="5" Width="100" />

        <Separator Grid.Column="3" Style="{StaticResource VerticalSeparatorStyle}" />

        <Button Grid.Column="4" TabIndex="3" Content="Save" Width="75" Margin="5" />
        <Button Grid.Column="5" TabIndex="4" Content="Cancel" Width="75" Margin="5" />
    </Grid>

    <Line HorizontalAlignment="Stretch" X2="1" Stretch="Fill" Stroke="Black" StrokeThickness="1" Margin="0,5" DockPanel.Dock="Top" />

    <Grid x:Name="ShellContentRoot">

        <!-- Current Page -->
        <ContentControl TabIndex="2" Content="{Binding CurrentAccount}" IsTabStop="False" />

        <!-- Search Results -->
        <local:PopupPanel local:PopupPanel.PopupParent="{Binding ElementName=ShellContentRoot}" />
    </Grid>
</DockPanel>

The only thing special about my SearchView is that the controls all set

TabIndex="{Binding Path=TabIndex, RelativeSource={RelativeSource 
    AncestorType={x:Type local:SearchView}}}"

Tab Order goes:

• UserControl Search ComboBox
• UserControl Search Text
• UserControl Search Button
• Database ComboBox
• ContentControl
• Save Button
• Cancel Button