While selecting some text in a scrivener document, I noticed a horizontal gap in the blue text selection colour between selected paragraphs. Hovering the cursor over this gap changed it to the cursor normally seen when dragging the split screen divider (but rotated by 90 degrees).
Clicking and dragging up/down at this point had the effect of expanding/contracting the space between several paragraphs below the cursor.
The document also became slow to edit (the cursor doing the spinning rainbow from time to time).
It’s possible that some text copied from the web invisibly introduced some formatting that Scrivener interpreted strangely.
(I resolved the issue by copying the text into a .txt document, pasting it into a new Scrivener document and deleting the original - sorry, no screenshots).