pass unsupported boxes verbatim without breaking them

src/TaglibSharp/Mpeg4/BoxFactory.cs

@@ -114,6 +114,8 @@ parent.Box is IsoSampleDescriptionBox &&
 				return new IsoFreeSpaceBox (header, file, handler);
 			else if (type == BoxType.Mean || type == BoxType.Name)
 				return new AppleAdditionalInfoBox (header, file, handler);
+			else if (type == BoxType.Chpl)
+				return new OpaqueBox (header, file, handler);
 
 			// If we still don't have a tag, and we're inside an
 			// ItemListBox, load the box as an AnnotationBox

src/TaglibSharp/Mpeg4/BoxTypes.cs

@@ -47,6 +47,7 @@ static class BoxType
 		public static readonly ReadOnlyByteVector Cmt = AppleTag.FixId ("cmt");
 		public static readonly ReadOnlyByteVector Cond = "cond";
 		public static readonly ReadOnlyByteVector Covr = "covr";
+		public static readonly ReadOnlyByteVector Chpl = "chpl";
 		public static readonly ReadOnlyByteVector Co64 = "co64";
 		public static readonly ReadOnlyByteVector Cpil = "cpil";
 		public static readonly ReadOnlyByteVector Cprt = "cprt";

src/TaglibSharp/Mpeg4/Boxes/OpaqueBox.cs

@@ -0,0 +1,105 @@
+//
+// OpaqueBox.cs: Provides a box implementation that preserves its
+// on-disk bytes exactly, with no re-parsing or re-rendering.
+//
+// This is used for atoms like the Nero chapter box (chpl) whose
+// internal format has multiple versions.  Re-rendering through the
+// normal Box pipeline can silently corrupt the data (e.g. version
+// and flags bytes).  By capturing the complete atom bytes on read
+// and returning them verbatim on render, we guarantee a lossless
+// round-trip.
+//
+// Copyright (C) 2026
+//
+// This library is free software; you can redistribute it and/or modify
+// it  under the terms of the GNU Lesser General Public License version
+// 2.1 as published by the Free Software Foundation.
+//
+// This library is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307
+// USA
+//
+
+using System;
+
+namespace TagLib.Mpeg4
+{
+	/// <summary>
+	///    This class extends <see cref="Box" /> to provide a box that
+	///    stores and renders its original on-disk bytes verbatim.
+	///    No header or data re-serialization is performed.
+	/// </summary>
+	public class OpaqueBox : Box
+	{
+		#region Private Fields
+
+		/// <summary>
+		///    The complete on-disk atom bytes (header + data).
+		/// </summary>
+		readonly ByteVector raw_bytes;
+
+		#endregion
+
+
+
+		#region Constructors
+
+		/// <summary>
+		///    Constructs and initializes a new instance of <see
+		///    cref="OpaqueBox" /> by reading the complete atom from
+		///    the file at the position described by the header.
+		/// </summary>
+		/// <param name="header">
+		///    A <see cref="BoxHeader" /> object containing the header
+		///    to use for the new instance.
+		/// </param>
+		/// <param name="file">
+		///    A <see cref="TagLib.File" /> object to read the contents
+		///    of the box from.
+		/// </param>
+		/// <param name="handler">
+		///    A <see cref="IsoHandlerBox" /> object containing the
+		///    handler that applies to the new instance.
+		/// </param>
+		/// <exception cref="ArgumentNullException">
+		///    <paramref name="file" /> is <see langword="null" />.
+		/// </exception>
+		public OpaqueBox (BoxHeader header, TagLib.File file, IsoHandlerBox handler) : base (header, handler)
+		{
+			if (file == null)
+				throw new ArgumentNullException (nameof (file));
+
+			file.Seek (header.Position);
+			raw_bytes = file.ReadBlock ((int)header.TotalBoxSize);
+		}
+
+		#endregion
+
+
+
+		#region Protected Methods
+
+		/// <summary>
+		///    Renders the box by returning the original on-disk bytes.
+		/// </summary>
+		/// <param name="topData">
+		///    Ignored.  The original bytes are returned as-is.
+		/// </param>
+		/// <returns>
+		///    A <see cref="ByteVector" /> containing the exact bytes
+		///    that were on disk when the box was read.
+		/// </returns>
+		protected override ByteVector Render (ByteVector topData)
+		{
+			return raw_bytes;
+		}
+
+		#endregion
+	}
+}