Is there a way to set the 'Z-index' or something like it? I have an animation I'd like to "slide out" from an element...

I have a left sidebar that looks good on desktop, and next to it, a visibility controlled window of extra options. When I open this, the animation (slide in from left) is great, but it currently goes OVER the sidebar on its way in.

Is there some sort of z-index or way to set the height priority for a given element so I can have another element intentionally overlap it? Then I can have it slide in from underneath the sidebar...