anchor.diff:  
     @override
     ui.EngineLayer addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
  -    builder.pushShaderMask(shader, maskRect.shift(layerOffset), blendMode);
  +    final Rect shiftedMaskRect = layerOffset == Offset.zero ? maskRect : maskRect.shift(layerOffset);
  +    builder.pushShaderMask(shader, shiftedMaskRect, blendMode);
       addChildrenToScene(builder, layerOffset);
       builder.pop();
       return null; // this does not return an engine layer yet.
  
positive.code: 
    @override
    ui.EngineLayer addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
      final Rect shiftedRect = rect.shift(layerOffset);
      builder.addTexture(
        textureId,
        offset: shiftedRect.topLeft,
  
negative.code: 
    @override
    ui.EngineLayer addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
      final Rect shiftedRect = layerOffset == Offset.zero ? rect : rect.shift(layerOffset);
      builder.addPlatformView(
        viewId,
        offset: shiftedRect.topLeft,
  