Here’s how I see this component on mobile with no other themes installed:
I didn’t have to resize to get this, and when I resize I don’t get the slide-in class at all…
It looks like you’re on Discourse 3.2, rather than the current version 3.3. There are updates to this component that rely on version 3.3, so you’re not getting those at the moment.
It’s possible that updating Discourse and the component will fix all of your issues, but if not there may be a conflict with other customizations.