Throw if markup contains unescaped close tag

src/Spectre.Console.Tests/Unit/MarkupTests.cs

@@ -0,0 +1,40 @@
+using System;
+using Shouldly;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public sealed class MarkupTests
+    {
+        [Theory]
+        [InlineData("Hello [[ World ]")]
+        [InlineData("Hello [[ World ] !")]
+        public void Should_Throw_If_Closing_Tag_Is_Not_Properly_Escaped(string input)
+        {
+            // Given
+            var fixture = new PlainConsole();
+
+            // When
+            var result = Record.Exception(() => new Markup(input));
+
+            // Then
+            result.ShouldNotBeNull();
+            result.ShouldBeOfType<InvalidOperationException>();
+            result.Message.ShouldBe("Encountered unescaped ']' token at position 16");
+        }
+
+        [Fact]
+        public void Should_Escape_Markup_Blocks_As_Expected()
+        {
+            // Given
+            var fixture = new PlainConsole();
+            var markup = new Markup("Hello [[ World ]] !");
+
+            // When
+            fixture.Render(markup);
+
+            // Then
+            fixture.Output.ShouldBe("Hello [ World ] !");
+        }
+    }
+}

src/Spectre.Console/Internal/Text/Markup/MarkupTokenizer.cs

@@ -91,6 +91,8 @@ namespace Spectre.Console.Internal
             {
                 var position = _reader.Position;
                 var builder = new StringBuilder();
+
+                var encounteredClosing = false;
                 while (!_reader.Eof)
                 {
                     current = _reader.Peek();
@@ -98,10 +100,34 @@ namespace Spectre.Console.Internal
                     {
                         break;
                     }
+                    else if (current == ']')
+                    {
+                        if (encounteredClosing)
+                        {
+                            _reader.Read();
+                            encounteredClosing = false;
+                            continue;
+                        }
+
+                        encounteredClosing = true;
+                    }
+                    else
+                    {
+                        if (encounteredClosing)
+                        {
+                            throw new InvalidOperationException(
+                                $"Encountered unescaped ']' token at position {_reader.Position}");
+                        }
+                    }
 
                     builder.Append(_reader.Read());
                 }
 
+                if (encounteredClosing)
+                {
+                    throw new InvalidOperationException($"Encountered unescaped ']' token at position {_reader.Position}");
+                }
+
                 Current = new MarkupToken(MarkupTokenKind.Text, builder.ToString(), position);
                 return true;
             }