Ubuntu – system does not power off on “poweroff”, just halts

I've installed Xubuntu 15.04 on a Lenovo IdeaCentre A740 QHD with a Haswell CPU (BIOS revision 00KT19AUS) and NVIDIA GeForce GTX 850A 2GB. It's mostly working, except when I do a shutdown or reboot, it doesn't actually turn off the power after quitting everything:

So I have to click the power button to actually turn it off.

I've kept the Windows 8.1 installation in case there's any future firmware. Before installing Xubuntu, I turned off Fastboot from Windows, then installed Xubuntu. Unfortunately, the UEFI BIOS didn't let me change boot order so that Ubuntu actually started as default. I tried bcdedit /set {bootmgr} path \EFI\ubuntu\shimx64.efi, tried turning off "quickboot" (whatever that is) in BIOS, tried the Boot-Repair program from a Live Session, and tried turning off SecureBoot, but still it would just boot Windows. I ended up, with the help of EricC^^ of #ubuntu on freenode, just switching around the .efi files to trick the boot manager into thinking Ubuntu was Windows:

cp /boot/efi/efi/boot/bootx64.efi{,.backup}
cp /boot/efi/efi/microsoft/boot/bootmgfw.efi{,.backup}
cp /boot/efi/efi/ubuntu/grubx64.efi /boot/efi/efi/boot/bootx64.efi
cp /boot/efi/efi/ubuntu/grubx64.efi /boot/efi/efi/microsoft/boot/bootmgfw.efi
cp /boot/efi/efi/ubuntu/grubx64.efi /boot/efi/efi/microsoft/boot/grubx64.efi
sudo vim /usr/lib/os-probes/mounted/efi/20microsoft
# and changed bootmgfw.efi to bootmgfw.efi.backup
update-grub

I don't know if any of this has a bearing on the shutdown trouble.

EDIT: Come to think of it, the reboot from the Xubuntu installation (when I was booted through a USB drive) didn't work either.

What I've tried so far to get it to shut down:

• acpi=off → no difference
• acpi=force → no difference
• install proprietary Nvidia drivers → that just made X not start with the message "bbswitch: No discrete VGA device found"
• various variations on sudo poweroff, sudo shutdown now, sudo shutdown -h now etc.

Also, if I reboot instead of shutdown, I get this psychedelic lightshow on my monitor and have to long-click the power button to turn it off:

If it's helpful, here's a journalctl –all output right after booting up and perhaps even better: journalctl -b -1 (journal from bootup to shutdown).

Also, perhaps related, I notice now that pressing the power button while logged in to XFCE turns the computer right off, even though I have XFCE power settings to "Ask when power button pressed" and "Do nothing" on any other buttons.

My /etc/systemd/logind.conf has no uncommented lines apart from the [Login] header.

There is a /usr/sbin/acpid process running as root.

EDIT: More revelations: Ctrl+Alt+Delete actually reboot fine from GRUB.

EDIT2: I've filed a bug report since this doesn't seem fixable with the regular tricks.

EDIT3: Solved with acpi=noirq and kernel 4.4 and newer.