How to force a usercontrol width to be equal designer width? - wpf

Question

I have a custom textbox in WPF and I want to change its width while resizing it in designer view. I used Binding ActualHeight but it doesn't work.
As the picture shows I resized the TextBox in design mode but it is still in the same width, not changed. How can I change that control's Width(only) property according to resizing?
DottedTextBox
<UserControl x:Class="Easy_Message.DottedTextBox"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="50" d:DesignWidth="400">
<Viewbox>
<TextBox x:Name="roottxt" Background="White"
BorderThickness="0" Width="{Binding ActualWidth,RelativeSource={RelativeSource TemplatedParent}}" Height="20">
<TextBox.Template>
<ControlTemplate TargetType="TextBox">
<Grid>
<Border x:Name="b" Width="{TemplateBinding Width}" Height="{TemplateBinding Height}"
BorderThickness="1,0,1,0" BorderBrush="#c2c4cb">
<Border BorderThickness="0,1,0,0" BorderBrush="#9da0aa">
<Border BorderThickness="0,0,0,1" BorderBrush="#d9dae0">
<DockPanel>
<TextBox x:Name="orgbox" VerticalContentAlignment="Center"
HorizontalAlignment="Left"
BorderThickness="0"
FontSize="{TemplateBinding FontSize}"
Background="White"
Width="{Binding Path=Width,ElementName=b,Converter={StaticResource wconv}}" SelectionBrush="{TemplateBinding SelectionBrush}"
Height="20">
</TextBox>
<Border x:Name="brdDot" Background="{TemplateBinding Background}" Grid.Column="1"
Width="16" Height="{TemplateBinding Height}" HorizontalAlignment="Right"
BorderThickness="0">
<Canvas HorizontalAlignment="Right"
Width="10" Height="6"
Margin="0,0,2.5,2.5">
<Rectangle Width="1.5" Height="1.5" Fill="Black">
<Rectangle.RenderTransform>
<TranslateTransform X="0" Y="2"/>
</Rectangle.RenderTransform>
</Rectangle>
<Rectangle Width="1.5" Height="1.5" Fill="Black">
<Rectangle.RenderTransform>
<TranslateTransform X="4" Y="2"/>
</Rectangle.RenderTransform>
</Rectangle>
<Rectangle Width="1.5" Height="1.5" Fill="Black">
<Rectangle.RenderTransform>
<TranslateTransform X="8" Y="2"/>
</Rectangle.RenderTransform>
</Rectangle>
</Canvas>
</Border>
</DockPanel>
</Border>
</Border>
</Border>
</Grid>
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding Path=IsMouseOver, ElementName=brdDot}" Value="True">
<Setter TargetName="brdDot" Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0,0.5" EndPoint="1,0.5">
<GradientStop Color="White" Offset="-0.1"/>
<GradientStop Color="#ccdfff" Offset="0.6" />
<GradientStop Color="White" Offset="1.1" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter TargetName="brdDot" Property="BorderThickness" Value="1,0,0,0" />
<Setter TargetName="brdDot" Property="BorderBrush" Value="#ccc9c9"/>
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</TextBox.Template>
</TextBox>
</Viewbox>