How to create Windows Phone 7 SMS textbox - windows-phone-7

Question

I'm looking to create a control that is similar to the "bubble" textbox found in the default SMS app. I was able to come close, creating a "bubble" shaped path, with a textbox inside it, but am having issues with the textbox sizing. I have set "AcceptsReturn" on the textbox to True. What I've noticed is that when the number of lines in the textbox increases, the textbox disappears from the view (under the SIP). This is not the way the SMS textbox behaves - it always seems to scroll up to show the latest line (whereas my control becomes hidden). Would appreciate any pointers...
[Update]
Managed to create this by customizing TextBox template:
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Rectangle Grid.Row="1" StrokeThickness="0" Fill="{TemplateBinding Background}"/>
<ContentControl Grid.Row="1" x:Name="ContentElement" HorizontalContentAlignment="Stretch" Margin="0" Padding="{StaticResource PhoneTouchTargetLargeOverhang}" VerticalContentAlignment="Stretch" FontSize="{StaticResource PhoneFontSizeNormal}" Background="Black" />
<Path Data="M161.25,90.75 L184.25,90.75 L184.25,100.75" HorizontalAlignment="Left" Grid.Row="0" Stretch="Fill" StrokeThickness="0" UseLayoutRounding="False" Width="23" RenderTransformOrigin="0.5,0.5" Fill="{TemplateBinding Background}" Margin="54.75,0,0,0">
<Path.RenderTransform>
<CompositeTransform Rotation="180"/>
</Path.RenderTransform>
</Path>
<Rectangle Grid.Row="2" Fill="{TemplateBinding Background}"/>
<TextBlock x:Name="txtTimeStamp" Grid.Row="2" TextWrapping="Wrap" Padding="{TemplateBinding Padding}" FontSize="{StaticResource PhoneFontSizeSmall}" Text="{Binding Timestamp, Converter={StaticResource dateTimeConverter}, Mode=TwoWay}">
<TextBlock.Foreground>
<SolidColorBrush Color="{StaticResource PhoneTextBoxForegroundColor}"/>
</TextBlock.Foreground>
</TextBlock>
</Grid>