navbar opening animation needs fine tuning

Steps to Reproduce

same example as in ZK-4840
open the nav "Users"
observer the animation closely

Current Result

during the animation the top item is cut half way

only at the very end the remaining part is "dragged down"

Expected Result

either
"rolldown" all items (without the final "shift")
or
"drag down" all subitems

but not combine both effects in this way

Debug Information

Workaround