✅ Dynamic Width

<!-- Dropdown etc. -->
<Grid Grid.Row="0"
        Grid.ColumnSpan="3">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="1*"/>
        <ColumnDefinition Width="1*"/>
    </Grid.ColumnDefinitions>

    <StackPanel Grid.Column="0">
        <ComboBox HorizontalAlignment="Center"
                    Margin="20"/>
    </StackPanel>
</Grid>