It appears that something (perhaps a change in the core?) has broken this component’s UX in mobile.
Specifically, when in the mobile breakpoint and a users clicks the chat icon to open the chat modal, the router sends the browser to /chat and thread categories are rendered as overlapped with the chat navigation.
Here’s two screenshots from two different users on different mobile browsers (Chrome and Safari), and I can also reproduce on Firefox for mobile.
For what it’s worth, I cannot reproduce this in a desktop browser that is in responsive design mode or mobile emulation mode. I think that has to do with the fact that the desktop doesn’t route to /chat when the chat icon is clicked on, but not 100% sure on that.
Could this be caused by the back to forum link? I don’t recall that being in earlier versions of the mobile breakpoint for Discourse but I could be wrong on that.