Add Add() method to ContainerSpan class (dotnet#904)

src/System.CommandLine.Rendering.Tests/ContainerSpanTests.cs

@@ -0,0 +1,21 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.IO;
+using FluentAssertions;
+using Xunit;
+
+namespace System.CommandLine.Rendering.Tests
+{
+    public class ContainerSpanTests
+    {
+        [Fact]
+        public void Container_span_supports_add_method()
+        {
+            var span = new ContainerSpan(new ContentSpan("content"));
+            span.Add(new ContentSpan(" with child"));
+
+            span.ContentLength.Should().Be("content with child".Length);
+        }
+    }
+}

src/System.CommandLine.Rendering/ContainerSpan.cs

@@ -60,5 +60,17 @@ public override void WriteTo(TextWriter writer, OutputMode outputMode)
                 _children[i].WriteTo(writer, outputMode);
             }
         }
+
+        public void Add(TextSpan child)
+        {
+            if (child == null)
+            {
+                throw new ArgumentNullException(nameof(child));
+            }
+
+            _children.Add(child);
+
+            RecalculateChildPositions();
+        }
     }
 }