How to Prevent auto-resizing in a custom activity designer?

Question

I hope you can help me with my problem. It's about a custom designer for a WF 4.0 activity, but the problem is essentially in the WPF of the designer. Some background

I've created a custom WorkFlow activity to send e-mails. For the custom designer for the activity, I've previously been using regular Textboxes for the "Subject" and "Body" of the e-mail, but I'd like to use the ExpressionTextBox to easily bind it to the InArguments of the activity. The ExpressionTextBoxes are in a grid, and this grid is on a StackPanel.

I've set the the MinWidth, MaxWidth and Margin of the ExpressionTextBoxes to fit with the other controls, and in the Visual Studio Designer (viewing the custom activity designer, not the actual WorkFlow) everything looks as it should.

<sapv:ExpressionTextBox Grid.Column="1" Grid.Row="2" Height="Auto" HorizontalAlignment="Right" Margin="4, 4, 4, 4" 
                        Expression="{Binding Path=ModelItem.Subject, Mode=TwoWay, Converter={StaticResource ArgumentToExpressionConverter}, ConverterParameter=In}"
                        ExpressionType="{x:Type TypeName=sys:String}" OwnerActivity="{Binding Path=ModelItem}" VerticalAlignment="Center" MaxWidth="176" MinWidth="175" />

The problem

When used, initially it also looks as it should, but when the ExpressionTextBoxes are edited, they shrink into being really small. When text is entered, the control expands to fit the text, until it reaches its MaxWidth. When the editing ends, it goes back to it's MaxWidth. I'd prefer if it stayed the same size, regardless of being in edit-mode or not.

If you can't see it, open the image here

What I've tried

I've mostly been doing WinForms, and I'm pretty inexperienced with WPF, so I don't know if there are some funky properties or other settings that I've missed. I've tried setting width-properties of the parent controls (StackPanel and Grid), I've tried setting just the width (no min/max), but it seems to shrink regardless of what I set.

If you would like more information or code, please don't hesitate to ask.

Update

As you can see in the comments to Maurices answer, I figured out how to avoid the problem by removing the horizontalAlignment property, and then using margins to align it to the right. But I'm not going to mark an answer, until there's an explanation of why this behaviour happened in the first place. My XAML was almost identical to what Maurice posted, so there must be something wrong elsewhere.

Answer 1

The XAML for the ExpressionTextBox looks fine to me and when I try the following designer it works just fine.

<sap:ActivityDesigner x:Class="WorkflowConsoleApplication2.MyActivityDesigner"
                      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                      xmlns:sys="clr-namespace:System;assembly=mscorlib"
                      xmlns:sap="clr-namespace:System.Activities.Presentation;assembly=System.Activities.Presentation"
                      xmlns:sapv="clr-namespace:System.Activities.Presentation.View;assembly=System.Activities.Presentation"
                      xmlns:sapc="clr-namespace:System.Activities.Presentation.Converters;assembly=System.Activities.Presentation">
  <sap:ActivityDesigner.Resources>
    <sapc:ArgumentToExpressionConverter x:Key="ArgumentToExpressionConverter" />
  </sap:ActivityDesigner.Resources>
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition/>
      <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
      <RowDefinition Height="30"/>
      <RowDefinition Height="30"/>
      <RowDefinition Height="30"/>
    </Grid.RowDefinitions>
    <Label Content="Subject"
           Grid.Row="2"
           Grid.Column="0"/>
    <sapv:ExpressionTextBox Grid.Column="1"
                            Grid.Row="2"
                            Height="Auto"
                            HorizontalAlignment="Right"
                            Margin="4, 4, 4, 4"
                            Expression="{Binding Path=ModelItem.Subject, Mode=TwoWay, Converter={StaticResource ArgumentToExpressionConverter}, ConverterParameter=In}"
                            ExpressionType="{x:Type TypeName=sys:String}"
                            OwnerActivity="{Binding Path=ModelItem}"
                            VerticalAlignment="Center"
                            MaxWidth="176"
                            MinWidth="175" />
  </Grid>
</sap:ActivityDesigner>

So I suspect the problem is possibly in your grid definition.