Fix issue in Markdown Parser with Two-Line Header 2 parsing

Issue was originally introduced when YAML front-matter support was added in e94daf5

Author: michael-hawker

Microsoft.Toolkit.Parsers/Markdown/Blocks/HeaderBlock.cs

@@ -11,6 +11,8 @@ namespace Microsoft.Toolkit.Parsers.Markdown.Blocks
 {
     /// <summary>
     /// Represents a heading.
+    /// <seealso href="https://spec.commonmark.org/0.29/#atx-headings">Single-Line Header CommonMark Spec</seealso>
+    /// <seealso href="https://spec.commonmark.org/0.29/#setext-headings">Two-Line Header CommonMark Spec</seealso>
     /// </summary>
     public class HeaderBlock : MarkdownBlock
     {

Microsoft.Toolkit.Parsers/Markdown/MarkdownDocument.cs

@@ -233,9 +233,9 @@ internal static List<MarkdownBlock> Parse(string markdown, int start, int end, i
                             realStartOfLine = startOfLine;
                             endOfLine = startOfLine + 3;
                             startOfNextLine = Common.FindNextSingleNewLine(markdown, startOfLine, end, out startOfNextLine);
-                        }
 
-                        paragraphText.Clear();
+                            paragraphText.Clear();
+                        }
                     }
 
                     if (newBlockElement == null && nonSpaceChar == '#' && nonSpacePos == startOfLine)

UnitTests/Markdown/Parse/HeaderTests.cs

@@ -6,6 +6,7 @@
 using System.Collections.Generic;
 using Microsoft.Toolkit.Parsers.Markdown.Blocks;
 using Microsoft.Toolkit.Parsers.Markdown.Inlines;
+using Microsoft.Toolkit.Parsers.Markdown;
 
 namespace UnitTests.Markdown.Parse
 {
@@ -56,6 +57,13 @@ public void Header_2()
         [TestCategory("Parse - block")]
         public void Header_2_Alt()
         {
+            /***
+             * Note: This is a tricky scenario because Header 2's 
+             * can be easily confused with thematic breaks, see specs:
+             * https://spec.commonmark.org/0.29/#setext-headings
+             * https://spec.commonmark.org/0.29/#thematic-breaks
+             ***/
+
             // Note: trailing spaces on the second line are okay.
             AssertEqual(CollapseWhitespace(@"
                 Header 2
@@ -64,6 +72,25 @@ Header 2
                     new TextRunInline { Text = "Header 2" }));
         }
 
+        [TestMethod]
+        [TestCategory("Parse - block")]
+        public void Header_2_Alt_NotHorizontalRule()
+        {
+            /***
+             * Note: This is a tricky scenario because Header 2's 
+             * can be easily confused with thematic breaks, see specs:
+             * https://spec.commonmark.org/0.29/#setext-headings
+             * https://spec.commonmark.org/0.29/#thematic-breaks
+             ***/
+
+            // Note: trailing spaces on the second line are okay.
+            AssertEqual(CollapseWhitespace(@"
+                Header 2
+                ---  "),
+                new HeaderBlock { HeaderLevel = 2 }.AddChildren(
+                    new TextRunInline { Text = "Header 2" }));
+        }
+
         [TestMethod]
         [TestCategory("Parse - block")]
         public void Header_3()