Reported on IRC, a user (Markworthen) has been intermittently experiencing the issue shown in the below annotated screenshot.
They are using Opera (53.0.2907.68) on Windows 10 64-bit. The issue is also present for them on Chrome (66.0.3359.181).
They have tried "refreshing the page; closing the browser and reopening it; and rebooting my desktop PC"
Perhaps T188839 is related?