setting different width/height for different videos will cause unexpected merge result.

[lzl124631x]

https://codepen.io/lzl124631x/pen/mdVgKBq

The video and camera are of different sizes (width x height). I read the sizes of them and set them to the streams, then I got the following unexpected result. The expected result would be that at least none of them are cropped.

Is there a good way to prevent mixer from stretching the videos? It by default fit them to the same width/height. But I'd prefer to add black margins and keep the original size.