re-add comment

Author: SpaceCheetah

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -654,7 +654,8 @@ private void InitializeFrame<TPixel>(
     {
         frame = image.Frames.AddFrame(previousFrame ?? image.Frames.RootFrame);
 
-        // If restoring to before first frame, restore to background. Same if first frame (previousFrameControl null).
+        // If the first `fcTL` chunk uses a `dispose_op` of APNG_DISPOSE_OP_PREVIOUS it should be treated as APNG_DISPOSE_OP_BACKGROUND.
+        // So, if restoring to before first frame, clear entire area. Same if first frame (previousFrameControl null).
         if (previousFrameControl == null || (previousFrame is null && previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToPrevious))
         {
             Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion();