Anyone able to help with a quick CSS transition?

Hey guys! I've finally got this effect working for my nav menu (On hover, black background slides down over link), but the only thing is it is not animating on MouseOut. What I mean is, if you mouse over DESIGN it will transition to black, but on mousing off of it the DIV just resets instantly

Would anyone mind helping with the fade out transition? Here is the jsfiddle link

Great design resource

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