Fix handling of case where default image isn't animated

Author: SpaceCheetah

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -234,8 +234,7 @@ public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, CancellationToken
                                 PngThrowHelper.ThrowMissingFrameControl();
                             }
 
-                            previousFrameControl ??= new((uint)this.header.Width, (uint)this.header.Height);
-                            this.InitializeFrame(previousFrameControl.Value, currentFrameControl.Value, image, previousFrame, out currentFrame);
+                            this.InitializeFrame(previousFrameControl, currentFrameControl.Value, image, previousFrame, out currentFrame);
 
                             this.currentStream.Position += 4;
                             this.ReadScanlines(
@@ -255,7 +254,7 @@ public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, CancellationToken
 
                             break;
                         case PngChunkType.Data:
-
+                            pngMetadata.DefaultImageAnimated = currentFrameControl != null;
                             currentFrameControl ??= new((uint)this.header.Width, (uint)this.header.Height);
                             if (image is null)
                             {
@@ -272,9 +271,12 @@ public Image<TPixel> Decode<TPixel>(BufferedReadStream stream, CancellationToken
                                 this.ReadNextDataChunk,
                                 currentFrameControl.Value,
                                 cancellationToken);
+                            if (pngMetadata.DefaultImageAnimated)
+                            {
+                                previousFrame = currentFrame;
+                                previousFrameControl = currentFrameControl;
+                            }
 
-                            previousFrame = currentFrame;
-                            previousFrameControl = currentFrameControl;
                             break;
                         case PngChunkType.Palette:
                             this.palette = chunk.Data.GetSpan().ToArray();
@@ -643,7 +645,7 @@ private void InitializeImage<TPixel>(ImageMetadata metadata, FrameControl frameC
     /// <param name="previousFrame">The previous frame.</param>
     /// <param name="frame">The created frame</param>
     private void InitializeFrame<TPixel>(
-        FrameControl previousFrameControl,
+        FrameControl? previousFrameControl,
         FrameControl currentFrameControl,
         Image<TPixel> image,
         ImageFrame<TPixel>? previousFrame,
@@ -652,15 +654,15 @@ private void InitializeFrame<TPixel>(
     {
         frame = image.Frames.AddFrame(previousFrame ?? image.Frames.RootFrame);
 
-        // if restoring to before first frame, restore to background
-        if (previousFrame is null && previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToPrevious)
+        // If restoring to before first frame, restore to background. Same if first frame (previousFrameControl null).
+        if (previousFrameControl == null || (previousFrame is null && previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToPrevious))
         {
             Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion();
             pixelRegion.Clear();
         }
-        else if (previousFrameControl.DisposeOperation == PngDisposalMethod.RestoreToBackground)
+        else if (previousFrameControl.Value.DisposeOperation == PngDisposalMethod.RestoreToBackground)
         {
-            Rectangle restoreArea = previousFrameControl.Bounds;
+            Rectangle restoreArea = previousFrameControl.Value.Bounds;
             Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(restoreArea);
             pixelRegion.Clear();
         }

src/ImageSharp/Formats/Png/PngMetadata.cs

@@ -83,6 +83,11 @@ private PngMetadata(PngMetadata other)
     /// </summary>
     public uint RepeatCount { get; set; } = 1;
 
+    /// <summary>
+    ///  Gets or sets a value indicating whether the default image is shown as part of the animated sequence
+    /// </summary>
+    public bool DefaultImageAnimated { get; set; }
+
     /// <inheritdoc/>
     public IDeepCloneable DeepClone() => new PngMetadata(this);