spectre.console/src/Spectre.Console/Rendering/Borders/Boxes/HeavyBoxBorder.cs

using System;

namespace Spectre.Console.Rendering
{
    /// <summary>
    /// Represents a heavy border.
    /// </summary>
    public sealed class HeavyBoxBorder : BoxBorder
    {
        /// <inheritdoc/>
        public override BoxBorder? SafeBorder => BoxBorder.Square;

        /// <inheritdoc/>
        public override string GetPart(BoxBorderPart part)
        {
            return part switch
            {
                BoxBorderPart.TopLeft => "┏",
                BoxBorderPart.Top => "━",
                BoxBorderPart.TopRight => "┓",
                BoxBorderPart.Left => "┃",
                BoxBorderPart.Right => "┃",
                BoxBorderPart.BottomLeft => "┗",
                BoxBorderPart.Bottom => "━",
                BoxBorderPart.BottomRight => "┛",
                _ => throw new InvalidOperationException("Unknown border part."),
            };
        }
    }
}