Not sure if that is related to AC or rather AUI team should look into this. Here's an issue that some of my users are experiencing recently:
I was able to reproduce it on my own, under IE as well while opening the Inline Dialog or choosing element on Dropdown2 list. In both cases, Add'ons IFRAME width starts to decrease to match the position of the opened dialog/dropdown element. I'm pretty sure there's no add-on's JS responsible for this as I don't do anything with the main IFRAME element.
As far as I can tell, it happens rather under Windows than other OS-es and under different browsers.
Below, are the reports of similar issues:
I'm pretty sure, this is something I can't fix or investigate further on my own so please somebody take a look. Thanks!