I've also noticed that XFCE's handling of hotplug monitors leaves much to be desired, and it also cheerfully ignores my preference of not suspending when I close the laptop lid (so I can use only the external monitor and an external keyboard when I have it plugged into my TV). Close the lid under XFCE and "BEEP!" it goes into suspend mode instead of doing nothing.
Fortunately, Cinnamon is just an apt-get away, handles both monitor hotplugging and closing the laptop lid sanely, and works the way I expect a desktop to work. I've settled on Xubuntu+Cinnamon as my go-to when setting up a desktop or laptop.
The setup I did was to just install Xubuntu, then:
apt install cinnamon
Once installed, I logged out, and then picked Cinnamon from the session selection menu (the little gear near the upper right corner). It comes right up, though it won't pick up your preferences from XFCE.
I hadn't realized there was now a Cinnamon spin - is that still in testing? It's not on Ubuntu's list of flavors.
Fortunately, Cinnamon is just an apt-get away, handles both monitor hotplugging and closing the laptop lid sanely, and works the way I expect a desktop to work. I've settled on Xubuntu+Cinnamon as my go-to when setting up a desktop or laptop.