Then fixed is maybe the wrong appoach.
theoretically you could use the .scroll feature to update the elements absolute top position, if the top changes, but ignore the scroll event when the left property changes.
Please look at this small and very simple example. http://jsfiddle.net/Hbkdt/
Notice: You can combine this with an animate feature, and debounce the window event, so that it does only fire every 30-50 milliseconds and then updates the values.
Animated and debounced example: http://jsfiddle.net/Hbkdt/1/
