(By GTK3 I guess. It seems strange that it would be toolkit-related, but there's clearly been a crusade somewhere in the system to flatten UI elements since that's been the trendy thing to do industry-wide for the last few years, so I wouldn't be surprised if border-y calls to GTK now result in "borders are 0px and we draw nothing", instead of "borders are BorderWidth in window color plus 2 bevel lines, so we'll draw that").
Anyway: in 16.04 (and earlier) the subwindows have proper framing, with a border between them and the panel edges (in both V and H layouts). In 18.04+ that's missing, so they directly abut "workspace" content, and that often causes visual bleedover with fullscreen apps. Can we please have the framing back? Even if it means SysMon having to draw them itself, if GTK3 can't be trusted to. Thanks.
(I can provide screencaps from 16 and 18+ if it's not clear).