i worked around BorderThickness, and looks a little better now

changed 1,1,1,1 to 1.3,1,1,1.3

almost ideal Tongue Out

here is the whole xaml:

<Border xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
     HorizontalAlignment="Left"
     Margin="45,187,0,0"
     VerticalAlignment="Top"
     Width="157"
     Height="22"
     BorderBrush="#A6B4C3"
     BorderThickness="1.3,1,1,1.3"
     CornerRadius="11,11,11,11" Background="#FFFFFFFF">
    <TextBox Margin="6,1,6,1" FontSize="12" BorderThickness="0,0,0,0"/>
</Border>