Slidedown jumping problem on hidden submenu

I have made a jQuery/css submenu, seen on the left side of the page b开发者_JAVA百科elow:

http://borviz.ro/exact-tours/inner.html

When the menu slides down, there is a jumping effect, which i cant figure out, why it is.

Can somebody help me? Thanks!

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apparently its costly animating the inline-block on the div.submenu > ul > li if you set this to display:block it’s a lot smoother