Fix PNG encoder blending.

Author: JimBobSquarePants

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -272,7 +272,17 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream, CancellationToken
                     ImageFrame<TPixel>? nextFrame = currentFrameIndex < image.Frames.Count - 1 ? image.Frames[currentFrameIndex + 1] : null;
 
                     frameMetadata = GetPngFrameMetadata(currentFrame);
-                    bool blend = frameMetadata.BlendMethod == PngBlendMethod.Over;
+
+                    // Determine whether to blend the current frame over the existing canvas.
+                    // Blending is applied only when the blend method is 'Over' (source-over blending)
+                    // and when the frame's disposal method is not 'RestoreToPrevious', which indicates that
+                    // the frame should not permanently alter the canvas.
+                    bool blend = frameMetadata.BlendMethod == PngBlendMethod.Over
+                                 && frameMetadata.DisposalMethod != PngDisposalMethod.RestoreToPrevious;
+
+                    // Establish the background color for the current frame.
+                    // If the disposal method is 'RestoreToBackground', use the predefined background color;
+                    // otherwise, use transparent, as no explicit background restoration is needed.
                     Color background = frameMetadata.DisposalMethod == PngDisposalMethod.RestoreToBackground
                         ? this.backgroundColor.Value
                         : Color.Transparent;

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -449,6 +449,7 @@ public void Encode_WithPngTransparentColorBehaviorClear_Works(PngColorType color
     [WithFile(TestImages.Png.APng, PixelTypes.Rgba32)]
     [WithFile(TestImages.Png.DefaultNotAnimated, PixelTypes.Rgba32)]
     [WithFile(TestImages.Png.FrameOffset, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Png.Issue2882, PixelTypes.Rgba32)]
     public void Encode_APng<TPixel>(TestImageProvider<TPixel> provider)
         where TPixel : unmanaged, IPixel<TPixel>
     {

tests/ImageSharp.Tests/TestImages.cs

@@ -76,6 +76,7 @@ public static class Png
         public const string FrameOffset = "Png/animated/frame-offset.png";
         public const string DefaultNotAnimated = "Png/animated/default-not-animated.png";
         public const string Issue2666 = "Png/issues/Issue_2666.png";
+        public const string Issue2882 = "Png/issues/Issue_2882.png";
 
         // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
         public const string Filter0 = "Png/filter0.png";