>I don't think we know [where the wide page problem originated] (short of cutting and pasting bits into a new message in a new thread)

Oh yes we do! Unlike the rest of you, the problem still persisted for me, because I was still on Page 1, having set my Display Preferences to 99 posts per page (a la lusy).

Setting preferences (temporarily!) to 1 post/page reveals that the problem is in fact bridget96's post with the long purple quote.

Sorry to dob you in, bridget, but I just couldn't let you weasel out of it like that.