Disable opacity mask on a TextBlock

I'm attempting to create a custom strikethrough line for WP7 (since it doesn't support the SL4/WPF strikethough).

I've done this by creating a 2px high rectangle, and placing it 33% from the bottom (as default for the Segeo WP font strikethrough).

However, this a rather strange issue with something that looks like a opacity mask.

The strikethrough is doing some masking on the textblock, and this is very much intendedly. So I'm looking for any ideas on how I can fix this.

Snippet of the XAML:

<TextBlock x:Name="ContentContainer"
           Text="{TemplateBinding Content}"
           Grid.Column="0"
           Foreground="{TemplateBinding Foreground}"
           HorizontalAlignment="Left"
           Margin="0,0,0,0"
           OpacityMask="{x:Null}"
           Padding="{TemplateBinding Padding}"
           FontSize="{StaticResource PhoneFontSizeLarge}" />

<Rectangle x:Name="CheckBorder"
           Fill="{StaticResource PhoneDisabledBrush}"
           Margin="0,26,0,15"
           OpacityMask="{x:Null}"
           Width="{Binding Width, ElementName=ContentContainer}"
           Height="2"开发者_如何学运维/>

As you can see I've tried setting the OpacityMask to {x:Null}, but alas, this have no effect at all.

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I'm not sure this is an OpacityMask problem - instead I think it's two sets of PhoneDisabledColor brushes combining together.

From the resources, it looks like PhoneDisabledColor is 40% opacity white

 <Color x:Key="PhoneDisabledColor">#66FFFFFF</Color>

So when you place two objects with this opacity on top of each other then you get the drawing effect you've seen - sections with stronger than 40% opacity.

There is a solution for strikethrough listed here - http://forums.silverlight.net/forums/t/19749.aspx

Edit/Update - having looked at that solution, I think it might show the same problem you have seen.

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Found via Strikeout text in Windows phone 7