Q:

WPF combobox size on drag GridSplitter

How can I anchor a combox box so that when I drag the splitter it will size automatically.
I tried using a stackpanel to no avail, and even setting the HorizontalAlignment and HorizontalContentAlignment to stretch; it does not work.  
I need the combo box to be as wide as the section is dragged.  In non wpf you just anchored the combo, in wpf that option is not available.
<StackPanel Orientation="Horizontal">
    <Label Content="View By" 
            Padding="5" 
            Height="30" />
    <ComboBox Name="ddSelector" 
            Height="25" 
            Width="auto" 
            HorizontalAlignment="Stretch" 
            VerticalAlignment="Stretch" 
            HorizontalContentAlignment="Stretch" 
            Margin="16,0,0,0" />
</StackPanel>

The stack panel is in row 0 and the grid splitter is in row 1.
<Grid.ColumnDefinitions>
    <ColumnDefinition Width="275"></ColumnDefinition>
    <ColumnDefinition Width="auto"></ColumnDefinition>
    <ColumnDefinition Width="350"></ColumnDefinition>
    <ColumnDefinition Width="auto"></ColumnDefinition>
    <ColumnDefinition Width="*"></ColumnDefinition>
</Grid.ColumnDefinitions>

<GridSplitter Width="5" 
    Margin="0,36,0,5" 
    BorderThickness="4" 
    BorderBrush="Transparent" 
    Grid.Column="1" 
    HorizontalContentAlignment="Center" 
    HorizontalAlignment="Center" 
    ResizeBehavior="PreviousAndNext"></GridSplitter>

A:

Use a Grid instead of a StackPanel. This will do.
<Grid>
   <Grid.ColumnDefinitions>
      <ColumnDefinition Width="Auto"/>
      <ColumnDefinition Width="*"/>
   </Grid.ColumnDefinitions>

   <Label ../>
   <ComboBox Grid.Column="1" HorizontalAlignment="Stretch" .../>

</Grid>

