I figured it out!
If you have Cohost Plus, there's a bit of text at the bottom of the composer modal that causes the post box to expand to its maximum width. My subscription lapsed, and when I renewed it, everything went back to normal.
This doesn't feel like intentional behavior - maybe a CSS change is due to make the box start at its max width for everyone?
I find that the box changing width while you're typing a post to be extremely jarring, to be honest! There's no other website I can really think of that does this. While I'm sure that's an intentional design choice, I don't see a real reason for it being there.
The post editor should probably be at max width regardless of the amount of content slapped in it, for consistency and getting rid of visual weirdness like this :0
bazelgeuse-apologist
Things were normal earlier today, but just now, the post composer modal... shrank? If you open the post composer in a new tab, it's the usual width, it's just the overlay modal.
Likewise, this is just desktop showing the issue. Mobile seems fine.
On Firefox, Linux Mint, no userscripts/custom styles, only extensions are uBlock Origin and Indie Wiki Buddy. Ctrl + F5 didn't resolve it, nor did updating Firefox.