This question has me completely stumped. Since it's in a browser extension, I can't post my code.
The issue is apparently that mouse events inside the iframe (hovering over a link, over text, selecting...) are off by about 200px, this includes right clicks as shown in this screenshot:
The red dot is the actual mouseclick location. Again, since this is an extension (I'm actually doing the JS part in Tampermonkey, it's easier) I can't try it out in other browsers so it might just be chrome.
The iframe is to contain third-party dynamically loaded content so I can't use a div or other element. It's also reused and is dynamically inserted on load.
This also doesn't happen the first time content is inserted instead it takes a few "cycles" to appear.
Any ideas?