OH WAIT. I KNOW WHAT'S HAPPENING HERE.
You're looking at it in a browser that doesn't support JPEG XL, so it's falling back to a WASM renderer, and I think it handles resizing differently.
This is totally my bad. I thought I was doing a good thing by adding that fallback renderer, but it isn't identical and it's giving people the wrong impression.
The lossless version is JPEG XL, hence the difference to AVIF.
In Firefox 157.0.1, the right border of the image moves by a pixel or two when I press "hold for lossless" while viewing the AVIF. I think it's the lossy AVIF that's a bit too small.
by the way, why not convert everything to png for comparisons to avoid browser support issues?
I also feel like sometimes the lossless image 'sticks' even after having released the button. Don't know how to reliably test that, though.