@TheAmplitwist Embarrassingly, it turns out that this banner comes from a userscript I installed, which provides tweaks to certain UI elements on SE. I'll try to not trouble everyone with such posts here in the future!

@TheAmplitwist Next to the search bar, Is that a small broom icon?

lol

I've never seen something like that, next to the bar

@VerónicaRmz. Yup, that's from this userscript: Flagging Assistant - tweaks for frequent flaggers. The popup banner that I was talking about is also from there.

@TheAmplitwist I see. Interesting to know, thanks.

@VerónicaRmz. No problem. Cheers!