Oh my god I'm such an idiot.. I didn't even know panic mode was a thing.
I think my confusion stemmed from the fact that all my open tabs disappeared from the taskbar as well. I figured they were closed, not just hidden. I've also never set any panic mode options, so I think maybe it just grabbed a random URL from browsing history for the new window?
Anyway, thank you very much for the information! I respectfully withdraw my request sir. I'm not sure how I fat-fingered F12, but that's the behavior for sure. It's Opera GX on Windows 10 Pro, in case you still want that information.