I've had this a few times this morning. (I've had it very occassionally in the past, but it's regular this morning.)
It's fairly self-explanatory - the autocomplete box remains visible after moving to the next segment and even when I switch to other programs. (Note: I'm running Ubuntu.)
It can only be fixed by typing something that induces a new autocomplete box to appear.
I've had this occassionally (not specially today) also with the translation memory unit information box.
Please fix!! :)
I just had this problem too yesterday for the first time (Mac-El Cap). Weird. I had to restart CT. Probably not related but this morning, CT refuses to load my project files. Everything was going so well last night... time for a ticket.