Merge pull request #3036 from jrlost/fix-3035

Fix VP8X reserved bytes use Position += 3 instead of Write, causing 3-byte truncation in alpha WebP images

Author: brianpopow

src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs

@@ -123,7 +123,10 @@ public void WriteTo(Stream stream)
         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);
 

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);
+    }
+}