They weren't there before. Before Gnome only had X. The _ [] X is new. I use Gnome Shell here at home, and I had actually not seen that. 🙂 Definitely interesting...
Though would you know whether upstream (Chrome or Chromium) hinders your screensaver from coming up? I'm using i3 and don't have any screen saver. If Chrome/Chromium does it, we might have a bug, -- if not, I agree with you this would really be a good feature.
I just realized that it will change for darker colors, except the threshold might be a little too low for my color. Perhaps make the color a bit lighter to match for transparent taskbars, or use transparency like other versions and the problem will be solved. Here is a picture, Thanks guys!