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?





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).



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

I've remembered the second aspect of this problem. On a lower-res display, i.e. 720p TV or laptop, what must be integer math is taking the already-bad border width of 1px, and scaling it down to 0.nn px, resulting in it not having ANY border at all.

Out of curiosity, is the root cause the same GTK3 issue that turned the window border interaction size from 4px to 1px? (Or something along those lines).
This widget REALLY needs a border, since unlike most applets it doesn't just present as an icon with margins already in place. Using xzoom on the 16.04 version shows it as 4px, which I think is reasonable as both minimum and maximum for a vertical panel layout. Can we just throw that in there as a constant? If whatever it's trying to read it from right now (or calculate it from) has been broken by GTK3, doing that would get it working well again with minimal effort.