The ATT settings frame no longer appears as a stand-alone window, but instead opens in the Blizzard Options frame.

As a result, and because the settings frame is now constrained to the Blizzard frame, almost all of the text in the first 4 tabs of ATT settings is badly rearranged or truncated, or both.

This sounds like a bad interaction with some UI addon.

After a little bit more investigating, I've narrowed it down to the Replace Blizzard Fonts checkbox in the General/Media tab in ElvUI. Turning this setting off fixes the problem. It actually seems to change nothing else on my setup, maybe as I have fonts dropped into a folder in World of Warcraft/retail.

I'll consider this an ElvUI issue and close it.