Bit late, but the best solution seems to be not to use Wayland and lower built in display to 1080p. As Xorg is the default on 18.04 anyway we'll go with that.
Connect monitor
Lower built in display to 1920x1080
Set scaling to 100% on both
Tested on driver nvidia-440
When disconnecting the monitor the resolution will set the built in display back to 4k so you shouldn't notice any difference between docked/undocked, except for the obvious loss of quality when watching videos etc (while docked).
This issue doesn't seem to get any better with 19.10 either :(
I'm back in Ubuntu! The issue was that the path to grub set in the BIOS after the firmware upgrade was wrong. Fixing the path fixed the issue.
To do this, press the F2 key while rebooting the Dell laptop, to bring up the BIOS interface.
Optional: In the menu on the left, first select System Configuration > SATA Operation, and select the AHCI radio button. (This step is required on my machine, because there are no RAID drivers installed for Ubuntu yet. If you have RAID drivers for Ubuntu, then you can choose RAID On instead.) Confirm the change of SATA Operation.
With the correct SATA setting already chosen, select Boot Sequence, and then click on the Add Boot Option button in the middle of the right pane. Name the new record (in my case — Ubuntu AHCI) and click on the [...] button to the right of the File Name field, choose a grub file for start-up. Select EFI > ubuntu > grub64.efi. Click OK.
Using the arrows by the list at the top right of the Boot Sequence pane, place your new Ubuntu Boot Option at the top of the list.
I had a new Boot Option with an unhelpful name (UEFI: THNSN5256GPU7 NVMe TOSHIBA 256 GB, Par) which had the same choice of file as Windows Boot Manager. I deleted this, and checked afterwards that I could still boot into Windows. Here is what my Boot Options look like now:
[✓] Ubuntu AHCI
[✓] Windows Boot Manager
Click Apply, confirm your changes, and then click Exit.
The machine should now boot into Ubuntu, just as it did before the firmware upgrade.
Best Answer
Dell firmware updates, if I am not wrong, are located in the
restrictedrepository.If you manually install Ubuntu 17.04, just type in a terminal
sudo add-apt-repository restricted. Then, search for Dell firmware updates in GNOME Software center. It will work like Ubuntu 16.04, where you still receive firmware updates, unless Dell really decided to abandon Ubuntu's firmware updates (which in this case, Ubuntu 16.04 will also be affected.)But the best choice, of course, is to keep your existing Ubuntu 16.04, install 17.04 alongside, so that even if you encounter problems, you can still fallback to 16.04 and ask something here.