WPF: Tab navigation broken with collapsed hyperlink

The problem is not the Hyperlink but the nested controls within the TextBlock. You could change it to

<TextBlock Visibility="Collapsed">               
  <TextBlock Text="MyText" />
</TextBlock>

and the Tab navigation would still be broken.

The solution is to use KeyboardNavigation.TabNavigation="Once" at the outer TextBlock:

<TextBlock KeyboardNavigation.TabNavigation="Once">
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Setter Property="Visibility" Value="Collapsed"/>
        </Style>
    </TextBlock.Style>
    <Hyperlink Focusable="False">
        <TextBlock Text="test" />
    </Hyperlink>
</TextBlock>

then everything works the way as intended. The problem is that the inner TextBlock gets the Focus, even if the outer Control it is collapsed. Setting KeyboardNavigation.TabNavigation to Once solves it as the whole Container and its childs get the focus only once. (MSDN)

@Gimno's answer put me on the right track, but I found that using KeyboardNavigation.TabNavigation="None" actually gives the top element focus only once (as you would expect from Once). Gimno's answer works because he/she also set Focusable="False" on the Hyperlink. With TabNav=None, you don't have to set Focusable on all the children controls.

Here's my application of this method (only the Button gets tab focus, not either textblock or the hyperlink):

<Button Command="{Binding ChangeSoundCommand}" Click="ChangeSoundClick" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="Transparent" BorderBrush="Transparent" BorderThickness="0" Padding="0" 
 KeyboardNavigation.TabNavigation="None">
    <Button.Template>
        <ControlTemplate>
            <Grid>
                <TextBlock Name="tb" HorizontalAlignment="Center" VerticalAlignment="Center" Visibility="Collapsed" >
                    <Hyperlink>Browse...</Hyperlink>
                </TextBlock>
                <TextBlock Name="w_content" Text="{Binding FilePath}" TextTrimming="CharacterEllipsis"  />
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger SourceName="w_content" Property="Text" Value="">
                    <Setter TargetName="tb" Property="Visibility" Value="Visible"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </Button.Template>
</Button>