Do you think the transition animation for those elements will ever be imported to UCP? They were so nice here on legacy wikis:
I've tried to make a CSS test with the ToC but I'm not satisfied, because it shrinks the elements inside the container, and not actually collapses the container. It seems the real good solution is using JS.
This transition is not present on current version of wikipedia, nor on Gamepedia, so it's more likely not gonna happening on UCP, isn't it? :(