GNOME in Ubuntu 18.04 and multi monitor setup
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In summary, Change Dock -> show on = all displays
After upgrading Ubuntu 16.04 LTS with Unity to Ubuntu 18.04 LTS you can switch off the Gnome desktop and go back to good 'ole Unity. Click the gear icon next to the Sign In button as shown below:
The good news is Unity is even better when suspending with an external monitor is attached that it was in 16.04.