This has never happened before, but now, whenever I click Enter twice to add an empty paragraph, a window pops up. How can I change this behaviour?
You’ve changed your writing mode to Scriptwriting. Go to:
Menu -> Format -> Scriptwriting -> click on Script Mode to return to Standard Text Mode
(Chap. 20, pg. 274 of the Scriv. Man.)
Ah, the mystery is resolved, thank you!
You’re welcome. 