Strange popup across desktop(s)

Question

CONCLUSION: It was actually a popup from Android Studio that was running in the background, and seemed to have somehow left a 'hotspot' on my screen that triggered the Structure menu. Not a virus, not browser dependent.

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Recently (last 2 days), I've been noticing a strange popup that appears on links in my browser (Firefox on MacOS and Ubuntu, and Chrome on MacOS):

It seems to appear just after I move off a link, and as far as I can tell, hitting the buttons does nothing.

I've turned off all Firefox plugins, but it still appears (and the cross-browser thing suggests it's not browser related). BUT, it doesn't appear in Safari, as far as I can determine.

I'm guessing it's some sort of js extension loaded on the sites, but it's appearing on quite diverse sites, and it seems to be above the browser-window: if I switch applications, it often remains on top.

I've tried Google reverse-image searching for it, and tried various Google search combinations to try to discover something about it, but have had no success.

Here are some other examples, one from AirBnB (Firefox):

and this one from dailymaverick.co.za, a South African news site (Chrome):

Can anybody tell me what this is?

Many thanks, C

Answer 1

Worked it out - it was a bug being caused by a floating window from Android Studio's Structure view that I had running in the background. For some reason, it seemed to have 'surfaced' an invisible rectangle that showed the menu, and I had misinterpreted it as a browser related thing. I only realised it probably wasn't that when it started appeared on my terminal too - then I worried for a while about weird trojans, and then discovered that it disappeared when I closed Android Studio.

Phew!