I think this might only be on smaller screens (I don’t remember), but when you click a post’s link icon it displays this and auto-highlights the URL. This is great because it makes it easy to copy with Ctrl+C.
However, sometimes this doesn’t work as you can see below. I’m not 100% sure, but I think it’s the case that it only correctly auto-highlights the URL the first time this action is taken (short of a full page refresh). After doing this once, if you navigate to another post or even just close the popover and click the link icon a second time on the same post, the link won’t get highlighted automatically.