In IOS8 Safari there is a new bug with position fixed.
If you focus a textarea that is in a fixed panel, safari will scroll you to the bottom of the page.
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Cleanly? no.
I recently had this problem myself with a fixed search field in a sticky header, the best you can do at the moment is keep the scroll position in a variable at all times and upon selection make the fixed element's position absolute instead of fixed with a top position based on the document's scroll position.
This is however very ugly and still results in some strange back and forth scrolling before landing on the right place, but it is the closest I could get.
Any other solution would involve overriding the default scroll mechanics of the browser.