iOS8 scrolling position:fixed element when interacting with a

When I am on iOS8 (this doesn't happen in iOS7, or anywhere else) and I interact with a inside a fixed position modal window, the modal element jumps up and down. Sometimes it stays a significant distance below. When I inspect the element, the inspector thinks the element is in the correct position, however the content is below.

Attached screenshots showing the strange behaviour:

Before select interaction: After select interaction: !

My dom structure is as follows:

> Body ~ 3000+pixels high
>> Overlay (position: fixed) ~ 100% viewport high
>> Modal window (position: fixed) ~ 400px high, 50% top, -200px margin top
>>> Form with <select>