Element position 'absolute' when attached to 'fixed' target #215

I'm using a target with 'position : fixed' but I've noticed something strange on element positioning.
I'd expect the element position to be always fixed (as far as the target is) but:

• when I enable it, tether sets the element position to 'absolute'
• if I scroll the body position changes to 'fixed' (after a short while)
• if I resize the window (or disable/enable) the position changes back to 'absolute'

On some browsers (IE, Edge, Safari) this is noticeable and annoying.
It happens also on tether.io in the examples with "click to show".