Flutter mask a circle into a container

ClipRRect worked best for me.

See reference video: https://www.youtube.com/watch?v=eI43jkQkrvs&vl=en

ClipRRect(
  borderRadius: BorderRadius.cirular(10),
  child: myContainerWithCircleWidget,
);

The simplest way to do this is to using an overlap and cliprect.

class OverlapSquare extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      height: 200,
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(10),
        color: Colors.red,
      ),
      child: ClipRect(
        clipBehavior: Clip.hardEdge,
        child: OverflowBox(
          maxHeight: 250,
          maxWidth: 250,
          child: Center(
            child: Container(
              decoration: BoxDecoration(
                color: Colors.white,
                shape: BoxShape.circle,
              ),
            ),
          ),
        ),
      ),
    );
  }
}

The OverFlowBox allows the circle to draw outside the bounds of its parent, and then the cliprect cuts it back to the edge.

Just an FYI - on my device I'm getting a tiny red line at the top & bottom of the white circle. I'm fairly sure that's a recently-introduced bug in flutter as I'm also seeing something similar if I put a white border around the container. I've raised an issue on github for that.