Comment 1 for bug 1682542

I think in general GNOME3 does quite well on multiple monitors, atleast in GNOME shell (don't think I have ever run Classic on my 3 monitor setup). I know a lot of work has gone into the mechanics of supporting multiple monitors over the years. Overview shows your windows grouped on the monitor that they are on. you can move windows between monitors with keyboard shortcuts (or context-menu) etc and lots of other little details have been worked on, like configuration etc.

I originally wrote the multiple monitor panels extension as an experiment with the hope of getting some of the bits upstreamed, but it probably raised more questions than it answered, I wasnt particularly happy with the result because it just didn't scale well past 2 monitors. And then I moved onto more important things like working on Ubuntu integration in upstream gnome.

I agree with Florian that it doesnt make sense to duplicate things across monitors, if
 they are not relevant to that specific monitor. This leaves two main items:
- the app-menus (application specific ones), although I really hope these go away eventually, with CSD decorations they would make more sense in the headerbar (which is actually what happens in classic and other GNOME based DE's I believe)
- the workspace thumbnail switchers

The latter are tricky when I duplicated the switchers across monitors in my extension, they only show ed the content of the monitor they were on. Straight away everyone wanted to be able change independent monitors to different workspaces ;( Doing a single switcher with a panorama view of all monitors just doesnt scale well if you have say 3-4 monitors. There is a reason these haven't been fixed yet, its very hard to come up with a design to works for everyone. This perhaps raises the bigger question of linux workspaces in the traditional sense perhaps dont scale well to multiple monitors in some senses!

Anyway back to your top panel question, having a hidden option to simply duplicate the top panel across monitors would be technically fairly easy to do (either as extension or as core code). However its not something I or the upstream devs are likely to work on.