WPF How to make control resize relative to next control - wpf

Question

I have defined the following XAML:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MinWidth="200" />
<ColumnDefinition MinWidth="200" />
<ColumnDefinition MinWidth="500" />
</Grid.ColumnDefinitions>
<StackPanel Grid.Column="0" Margin="2,2,5,2">
<GroupBox Header="Computer">
<DockPanel>
<ComboBox MinWidth="100" Name="cmbComputerNames" IsEditable="True" DockPanel.Dock="Left" HorizontalAlignment="Stretch" Width="auto" />
<Button Content="Connect" Name="bConnect" Width="65" HorizontalAlignment="Right" />
</DockPanel>
</GroupBox>
</StackPanel>
<Button Grid.Column="1" Content="Two" Margin="1,2,5,2" />
<Button Grid.Column="2" Content="Three" Margin="1,2,2,2" />
<GridSplitter Height="100" Width="4" Grid.Column="0"/>
<GridSplitter Height="100" Width="4" Grid.Column="1"/>
</Grid>
So, the left grid column is resizable. I want the "Connect" button to remain right-aligned and with same width. The combobox however, should be left-aligned and the width should grow as the column is resized, so the distance to the connect button remains the same.
Doesn't work:
Can anyone tell me how I can achieve that?

Answer 1

Since this is too long for a comment
Replace DockPanel with Grid and try this:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<GroupBox Header="Some text here">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ComboBox MinWidth="100"/>
<Button Grid.Column="1" Content="A button" Margin="5"/>
</Grid>
</GroupBox>
<GridSplitter Grid.Column="1" Grid.RowSpan="2" ResizeDirection="Columns" ResizeBehavior="PreviousAndNext" Width="10"/>
<Button Content="some button" Grid.Column="2"/>
</Grid>
#Andy, if you could produce an answer then I will delete mine.