Responsive Menu Transition on Toggle – twenty twelve

Hi all,

I cannot work this one out! I think this is partly down to not entirely understanding the menu structure but also the CSS.

The navigation menu of Twenty Twelve at screen sizes less than 600px by default just appears when the menu is toggled on. Is there a way to for the drop down menu to transition to its full (unspecified) height as if it is sliding in?

I haven’t posted the code as it’s default Twenty Twelve (although I am going to make the change in a child theme).

Thanks in advance.