From 2edab61535c53ae9c665b931c78fcf77cc55407c Mon Sep 17 00:00:00 2001
From: John Fredrickson <john.fredrickson@leadventure.com>
Date: Tue, 23 Dec 2025 13:31:23 -0600
Subject: [PATCH 1/2] Write into stream instead of simply just advancing it.

---
 src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs b/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
index 491f71650..7cec7c1db 100644
--- a/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
+++ b/src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
@@ -123,7 +123,10 @@ internal readonly struct WebpVp8X : IEquatable<WebpVp8X>
         long pos = RiffHelper.BeginWriteChunk(stream, (uint)WebpChunkType.Vp8X);
 
         stream.WriteByte(flags);
-        stream.Position += 3; // Reserved bytes
+
+        Span<byte> reserved = stackalloc byte[3];
+        stream.Write(reserved);
+
         WebpChunkParsingUtils.WriteUInt24LittleEndian(stream, this.Width - 1);
         WebpChunkParsingUtils.WriteUInt24LittleEndian(stream, this.Height - 1);
 

From adf677a829590b02a72753e741506b9615c635c5 Mon Sep 17 00:00:00 2001
From: Brian Popow <popow@gmx.de>
Date: Sun, 28 Dec 2025 17:25:08 +0100
Subject: [PATCH 2/2] Add test for writing reserved bytes in WebpVp8X

---
 .../Formats/WebP/WebpVp8XTests.cs             | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 tests/ImageSharp.Tests/Formats/WebP/WebpVp8XTests.cs

diff --git a/tests/ImageSharp.Tests/Formats/WebP/WebpVp8XTests.cs b/tests/ImageSharp.Tests/Formats/WebP/WebpVp8XTests.cs
new file mode 100644
index 000000000..78be0bf9f
--- /dev/null
+++ b/tests/ImageSharp.Tests/Formats/WebP/WebpVp8XTests.cs
@@ -0,0 +1,26 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Webp.Chunks;
+
+namespace SixLabors.ImageSharp.Tests.Formats.WebP;
+
+[Trait("Format", "Webp")]
+public class WebpVp8XTests
+{
+    [Fact]
+    public void WebpVp8X_WriteTo_Writes_Reserved_Bytes()
+    {
+        // arrange
+        WebpVp8X header = new(false, false, false, false, false, 10, 40);
+        MemoryStream ms = new();
+        byte[] expected = [86, 80, 56, 88, 10, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 39, 0, 0];
+
+        // act
+        header.WriteTo(ms);
+
+        // assert
+        byte[] actual = ms.ToArray();
+        Assert.Equal(expected, actual);
+    }
+}