Window Height, Change Div CSS Class

Is this possible?

I have a website, two divs.
The right div is fixed position.

However when the user browser window, is less than the height of the div, i need it to be no longer fixed, but to scroll with the rest of the page.

I imagine the logic to be the following:

-detect div#right height

-if window-height is less than div#right height. change div class to xxx
-if window-height is greater than div#right height. div class stays at yyy

(must update window-height everytime window is resized)

any idea on this one?