I see an issue in page:
https://codepen.io/anon/pen/eWyEGR?editors=1010

OS: Ubuntu 16.04 (64-bit)
Browser and version: Chromium Version 58.0.3029.81

Steps to reproduce:

1. open codepen example
2. scroll one screen down
3. zoom image
4. see that it has wrong position

Use-case: fixed chat container with images in messages that can be zoomed when page is scrolled down.
Mb fixed position for zoomed image?