To have it disappear (a workaround):
right click on the window and "reload frame".
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I keep getting this pop-up whenever I login or simply f5 the page. Neither button is responsive. Other websites I can close it though.
I recall I clicked "Deny" the first time around and I suspect that's the reason it's frozen now. On other sites I then went for "Allow" and the notification went away for good there.