I don't think that expanded in Zotero 6 either, did it? I can see that it'd be nice to have that extensible, but it's pretty clearly designed as a fixed width field (it also doesn't adjust based on panel width) and I'm guessing having individual elements (rather than whole panels) adjustable would be a bit messy.
Adam Smith, is there any way users can appeal to you or others at Zotero to look into the possibility of modifying this feature. I am wondering from your comments that it might be "messy" to change this whether it is difficult, impossible, or maybe something that just never come up. It would be good to know if it can be improved.
Zotero core devs read every thread here -- lack of reply isn't same as lack of attention