Drop a leading byte order mark when adding frontmatter to a document [patch]

Frontmatter.Test/DelimiterLineTests.cs

@@ -184,6 +184,27 @@ public void AddFrontmatter_OpeningDelimiterHasTrailingWhitespaceOrBom_MergesInto
 		Assert.AreEqual($"---{Nl}title: A{Nl}author: B{Nl}---{Nl}Body{Nl}", result);
 	}
 
+	[TestMethod]
+	public void AddFrontmatter_BomPrefixedDocumentWithoutHeader_LeavesNoBomInTheBody()
+	{
+		string input = $"# Heading{Nl}{Nl}text{Nl}";
+
+		string result = Frontmatter.AddFrontmatter(input, new Dictionary<string, object> { ["title"] = "T" });
+
+		Assert.AreEqual($"---{Nl}title: T{Nl}---{Nl}# Heading{Nl}{Nl}text{Nl}", result);
+		Assert.AreEqual($"# Heading{Nl}{Nl}text", Frontmatter.ExtractBody(result));
+	}
+
+	[TestMethod]
+	public void AddFrontmatter_BomPrefixedDocumentOpeningWithBlankLines_LeavesNoBomInTheBody()
+	{
+		string input = $"{Nl}{Nl}# Heading{Nl}";
+
+		string result = Frontmatter.AddFrontmatter(input, new Dictionary<string, object> { ["title"] = "T" });
+
+		Assert.AreEqual($"---{Nl}title: T{Nl}---{Nl}# Heading{Nl}", result);
+	}
+
 	[TestMethod]
 	public void HasFrontmatter_DelimiterWithoutLineEnding_IsNotFrontmatter()
 	{

Frontmatter/Frontmatter.cs

@@ -137,7 +137,7 @@
 		}
 
 		List<Dictionary<string, object>> frontmatterObjects = ExtractFrontmatterObjects(input, out _);
 		return frontmatterObjects.Count > 0 ? frontmatterObjects.First() : null;
 	}
 
 	/// <summary>
@@ -284,7 +284,7 @@
 		}
 
 		// Then add any remaining properties that weren't in the standard order
 		foreach (KeyValuePair<string, object> property in frontmatter)
 		{
 			if (!sortedFrontmatter.ContainsKey(property.Key))
 			{
@@ -334,7 +334,7 @@
 				continue;
 			}
 
 			if (YamlSerializer.TryParseYamlObject(section, out Dictionary<string, object>? frontmatterObject) && frontmatterObject != null)
 			{
 				frontmatterObjects.Add(frontmatterObject);
 			}
@@ -482,7 +482,7 @@
 			lines.Add((start, i));
 			if (c == '\r' && i + 1 < input.Length && input[i + 1] == '\n')
 			{
 				i++;
 			}
 
 			start = i + 1;
@@ -496,13 +496,15 @@
 	/// Trims a document body for output, dropping the blank lines before its first content line and the
 	/// whitespace after its last. Unlike <see cref="string.Trim()"/>, it keeps the first content line's
 	/// indentation, so a body that opens with an indented code block or nested list content survives.
+	/// A leading byte order mark is dropped too: it is not whitespace, so it would otherwise be carried
+	/// past a newly written header and hide the first line from markdown renderers.
 	/// </summary>
 	/// <param name="body">The body to trim.</param>
-	/// <returns>The body without leading blank lines or trailing whitespace.</returns>
+	/// <returns>The body without a byte order mark, leading blank lines or trailing whitespace.</returns>
 	private static string TrimBody(string body)
 	{
-		int start = 0;
-		for (int i = 0; i < body.Length; i++)
+		int start = OpeningLineStart(body);
+		for (int i = start; i < body.Length; i++)
 		{
 			char c = body[i];
 			if (c == '\n')
@@ -552,7 +554,7 @@
 		}
 
 		// Then, add properties from dictionary b that don't exist in a
 		foreach (KeyValuePair<string, object> kvp in b)
 		{
 			if (!combinedFrontmatterObject.ContainsKey(kvp.Key))
 			{