Having found the magic incantations* that allowed me to force the use of the spiffy new amdgpu driver on 4.14.<something> instead of the radeon driver on my laptop, a quick try-out of 4.16 showed me a nice black, unused external display. Sigh.
I would report the issue (which might easily be user ignorance on my part), but the bug-reporting process through Ubuntu is worse than completing a tax-return. No doubt it is made deliberately hard to keep out the riff-raff (like me), but it does mean stuff does go unreported, and even if reported, often unresolved, as I can't do the necessary bisection** to find out which particular patch/update caused the problem.
*GRUB2 kernel boot options in /etc/default/grub
**I know why this needs to be done, but I don't have a spare test device I can do all the necessary work on, which is frustrating. I guess I need to get slicker at building and booting from a USB key to test different kernel/systemd/driver versions.