I guess I was too tired yesterday, here's what happened:
I downloaded the archive I had previously uploaded and tried it out locally. It only worked in firefox, chrome and safari didn't hide the element outside the iFrame. And didn't check closer and wrote my comment here.
Now I checked again, and only now I remember that before uploading the archive here I had also tested it by uploading it to a server - where it does work fine. Locally, it doesn't work so well, because of this error:
Some browser-safeguarding together with not-enough-sleep tripping me up there