System Monitor panel applet framing broken in 18.04

(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).

I'm not 100% clear - are you referring to a thin 1px border or the gap between each monitor in the applet?

16.04

Screenshot_20190827_102709

18.04

Screenshot_20190827_102729

Besides the GTK3 toolkit, it could be the applet's GTK3 code itself or possibly the themes need additional styles.

No, I mean the framing in the other direction, i.e. perpendicular to the monitor layout, and thus adjacent to the desktop. (The inter-monitor framing also changed, doubtless for the same reason, and although I prefer 16.04's version of that as well it's much less important).

16.04
sysmon1604

18.04
sysmon1904

(There's an unrelated bug in those screenies in the netspeed applet - just ignore that).