Return every stacked block's properties from ExtractFrontmatter [patch]

Frontmatter.Test/StackedBlockPreservationTests.cs

@@ -41,6 +41,31 @@ public void AddFrontmatter_StackedBlocksRepeatAKey_FirstBlockWins()
 		Assert.AreEqual("First", frontmatter["title"]);
 	}
 
+	[TestMethod]
+	public void ExtractFrontmatter_TwoStackedBlocks_ReturnsPropertiesFromEveryBlock()
+	{
+		const string input = "---\ntitle: Hello\n---\n---\nauthor: Jane\n---\n# Body\n";
+
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(input);
+
+		Assert.IsNotNull(frontmatter);
+		Assert.AreEqual("Hello", frontmatter["title"]);
+		Assert.AreEqual("Jane", frontmatter["author"], "The second block is header to ExtractBody, so ExtractFrontmatter must return it");
+		Assert.AreEqual("# Body", Frontmatter.ExtractBody(input));
+	}
+
+	[TestMethod]
+	public void ExtractFrontmatter_StackedBlocksRepeatAKey_FirstBlockWins()
+	{
+		const string input = "---\ntitle: First\n---\n---\ntitle: Second\nauthor: Jane\n---\nbody\n";
+
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(input);
+
+		Assert.IsNotNull(frontmatter);
+		Assert.AreEqual("First", frontmatter["title"]);
+		Assert.AreEqual("Jane", frontmatter["author"]);
+	}
+
 	// The first block is always read as frontmatter, so an unparseable first block followed by one that
 	// parses is the case where the unreadable text sits in the header rather than the body.
 	private const string UnreadableFirstBlock = "---\nkey: [unclosed\n---\n---\ntitle: T\n---\nbody\n";

Frontmatter/Frontmatter.cs

@@ -126,6 +126,10 @@
 	/// Extracts frontmatter from a markdown document.
 	/// </summary>
 	/// <param name="input">The markdown document content as a string.</param>
+	/// <remarks>
+	/// Consecutive frontmatter blocks form one header, so their properties are combined, and the first
+	/// block to define a key wins.
+	/// </remarks>
 	/// <returns>A dictionary containing the frontmatter properties, or null if no frontmatter is found.</returns>
 	/// <exception cref="ArgumentNullException">Thrown when input is null.</exception>
 	public static Dictionary<string, object>? ExtractFrontmatter(string input)
@@ -137,8 +141,10 @@
 			return null;
 		}
 
-		List<Dictionary<string, object>> frontmatterObjects = ExtractFrontmatterObjects(input, out _);
-		return frontmatterObjects.Count > 0 ? frontmatterObjects.First() : null;
+		// Stacked blocks are one header, as AddFrontmatter and ExtractBody treat them, so fold them all
+		// and let an earlier block win a repeated key.
+		Dictionary<string, object> frontmatter = CombineAllFrontmatterObjects(ExtractFrontmatterObjects(input, out _));
+		return frontmatter.Count > 0 ? frontmatter : null;
 	}
 
 	/// <summary>
@@ -301,7 +307,7 @@
 		}
 
 		// Then add any remaining properties that weren't in the standard order
 		foreach (KeyValuePair<string, object> property in frontmatter)
 		{
 			if (!sortedFrontmatter.ContainsKey(property.Key))
 			{
@@ -351,7 +357,7 @@
 				continue;
 			}
 
 			if (YamlSerializer.TryParseYamlObject(section, out Dictionary<string, object>? frontmatterObject) && frontmatterObject != null)
 			{
 				frontmatterObjects.Add(frontmatterObject);
 			}
@@ -499,7 +505,7 @@
 			lines.Add((start, i));
 			if (c == '\r' && i + 1 < input.Length && input[i + 1] == '\n')
 			{
 				i++;
 			}
 
 			start = i + 1;
@@ -631,7 +637,7 @@
 		}
 
 		// Then, add properties from dictionary b that don't exist in a
 		foreach (KeyValuePair<string, object> kvp in b)
 		{
 			if (!combinedFrontmatterObject.ContainsKey(kvp.Key))
 			{