I\'ve looked into stackoverflow and it seems that position:fixed in an iframe for iOS mobile devices has been an issue in the past but have yet to have an answer.
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In Chrome and Safari on iOS it seems the issue here is not the fixed position, but with the height of the iframe. From another SO answer: "Starting in version 4.2.1 of Safari iOS, and still going as of 4.3.3, the entire content of an iFrame is forcibly shown."
These other posts might help you: