This is an irritation bug only but I wanted to make sure its being looked at.

Set the AcceptReturn property to True in the TextBox control and the VerticalAlignment property to Top.

The text box should expand vertically as neccessary.  Pressing <ENTER> will not expand it until a new character is typed.  The blank line should be included in the calculation for the control's hieght.

Here's the XAML no code behind is required. 

<UserControl x:Class="SilverlightBugs.Page"
    xmlns="http://schemas.microsoft.com/client/2007"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Width="400" Height="300">
    <Grid x:Name="LayoutRoot" Background="White">
        <TextBox AcceptsReturn="True" VerticalAlignment="Top" />
    </Grid>
</UserControl>
 

Hi:

  This bug has been fixed in latest bits.

Thanks for your reporting.

Sincerely,
Allen Chen
Microsoft Online Community Support

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i think tehre is another bug, dosen't work in the fullscreenmode