Aha! Finally found the culprit. After noticing no horizontal scrollbar in another app, I began to think it must be something system-wide. I haven't done _too_ much modification at that level, but I remember a while back I added some some GTK CSS modifications in `~/.config/gtk-3.0/gtk.css`. Eventually found some settings that were matching against `paned *` and when I took that out, _voila!_ my horizontal scrollbars are back!