You mean, before today?
The problem was with details. There's only ONE select box that used that feature before. Because I wrote the text content for the new select box myself, I added details for it as well -- and I noticed that they were broken, i.e. they were showing in a single line and deforming the entire select box. So I rewrote the code to hide details, then force the dropdown to the current width, and reenable them.
Hmm, maybe I should do that on $dd rather than $items...