I'm not going to lie, the whole thing is a
mess quagmire. "Responsive width" is achieved through CSS hacks, and here we see the hacks used by
Magnific Popup conflict with the hacks used by my add-on even though they both use
the same technique. This is nobody's fault, it's just an unfortunate situation.
I have tested a few
more hacks solutions locally and I believe the simplest way to fix this issue in the near future would be to edit the
xengallery_media_preview.css file (copy/paste the full name in the Search box at the top right of the admin panel) and add the following snippet at the top:
Code:
.mfp-iframe-scaler>div>div>iframe
{
position: fixed !important;
}
It works for me on my local board, please try it out and if you confirm it works for your use case I'll look into bundling this template modification in the add-on.