From 51cd15cb2d07127899089de5e5146088414845ed Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Tue, 6 Oct 2026 04:24:02 +0000
Subject: [PATCH] Drop a leading byte order mark when adding frontmatter to a
 document [patch]

AddFrontmatter on a document with no header trimmed it with TrimBody, which
does not treat U+FEFF as whitespace, so the BOM landed after the new header at
the start of the body and hid a first-line heading from markdown renderers.
TrimBody now skips a leading BOM before it trims.

Fixes ktsu-dev/Frontmatter#160

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QiAEodt8KsqJyMYzdfoAfo
---
 Frontmatter.Test/DelimiterLineTests.cs | 21 +++++++++++++++++++++
 Frontmatter/Frontmatter.cs             |  8 +++++---
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/Frontmatter.Test/DelimiterLineTests.cs b/Frontmatter.Test/DelimiterLineTests.cs
index 198892b..baa4c9a 100644
--- a/Frontmatter.Test/DelimiterLineTests.cs
+++ b/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()
 	{
diff --git a/Frontmatter/Frontmatter.cs b/Frontmatter/Frontmatter.cs
index 76b1f83..1f59f33 100644
--- a/Frontmatter/Frontmatter.cs
+++ b/Frontmatter/Frontmatter.cs
@@ -496,13 +496,15 @@ private static bool IsFollowOnBlock(string block, List<(int Start, int End)> lin
 	/// 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')