After pressing screenshot key, I sometimes cannot escape spectacle screenshot interface with esc or do the shot with enter key. I can only end this with double clicking on the box. I have a dual monitor setup like this bug, which I think might be related: https://bugzilla.redhat.com/show_bug.cgi?id=1446846 . I don 't know a reliable way to reproduce this. For example, it may behave like this once and become normal afterwards. I usually see this at about once a week.
Happens to me too at times, I use notebook display and 2 external displays when it happens. Never encountered it on single screen (notebook undocked).
*** This bug has been marked as a duplicate of bug 374009 ***