GridLength.Start does not take into account Padding & Margin

[RolandUI]

When either horizontal Padding or Margin is set on the Row, the content doesn't fit in the table and the horizontal scrollbar becomes visible.

GridLength.Start should take them into account.

20250224-1552-01.4300016.mp4

Repo:

<TreeDataGrid Source="{Binding Source}">
    <TreeDataGrid.Styles>
        <Style Selector="TreeDataGridRow > Border#RowBorder">
            <Setter Property="Padding" Value="1" />
            <!-- <Setter Property="Margin" Value="1" /> -->
        </Style>
    </TreeDataGrid.Styles>
</TreeDataGrid>

public partial class MainWindowViewModel : ViewModelBase
{
    private ObservableCollection<Person> _people = new()
    {
        new Person { FirstName = "Eleanor", LastName = "Pope", Age = 32 },
        new Person { FirstName = "Jeremy", LastName = "Navarro", Age = 74 },
        new Person { FirstName = "Lailah ", LastName = "Velazquez", Age = 16 },
        new Person { FirstName = "Jazmine", LastName = "Schroeder", Age = 52 },
    };
    
    public MainWindowViewModel()
    {
        
        Source = new FlatTreeDataGridSource<Person>(_people)
        {
            Columns =
            {
                new TextColumn<Person, string>("First Name", x => x.FirstName, width: GridLength.Auto),
                new TextColumn<Person, string>("Last Name", x => x.LastName, width: GridLength.Star),
                new TextColumn<Person, int>("Age", x => x.Age, width: GridLength.Auto),
            },
        };
    }

    public FlatTreeDataGridSource<Person> Source { get; }
}