See attachment 15555 for video.
To reproduce, bring your pointer down from above the link so that the Hovercard appears with the arrow touching/overlapping the link ( https://i.imgur.com/0qYUgFQ.png ), then move the pointer up and down around the area-of-intersection (near where the hyperlink-underline appears)
Seen in Firefox and Chrome, on Linux.
Not reproducible on Windows 7 or MacOS, according to Eduard Braun and Prateek Saxena.
Possibly the (discussed elsewhere) changes to the default timing (from 150ms to 500ms), will help resolve this.