See the revision histories here:
The long code blocks page-stretch, as opposed to receiving their own max width and scrollbar (as they do on the Question page).
This caused me to misunderstand the "Side-by-Side" view's diff. It looked like an "answer" had been edited into the question, but there was no green text.
The right-hand side of the diff is completely off screen.
This is fixed.
When working on review queues, I switched from using fixed width tables to flexbox for rendering the side-by-side HTML diff, and apparently <table> elements are just as stubborn about <pre> element width as flexbox, if not more so. Posts with wide enough code blocks caused the posts to explode outside of their containers.
Incidentally, we had a previously reported but similar problem with initial revisions (and possibly inline changes) for posts with very long code blocks, reported at Revision page now fails the x-overflow.
I decided to go big here and just make the whole page responsive, replacing the table with carefully calculated header rows that collapse in a reasonable but Designed-by-a-Developer™ way.
<pre>elements to be collapsable when they're sitting inside a<table>. I'm gonna evaluate moving that page from being a massive table.<table>). example