OP's answer removed from the question and edited slightly:
I found the solution on this webpage (in
It tells us to
sudo -H gedit /etc/default/grub
then find this line
and add a parameters
acpi_backlight=intel, so the
edited line reads:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi= acpi_backlight=intel"
save the file, and then to make changes effective, run
finally you turn off / restart your monster and it should work!!*
I should mention that I still have the
/usr/share/X11/xorg.conf.d/20-intel.conf file as described in the
first article I
linked to in my question.**
* CW post author's note: You may want to try the boot parameters individually first, rather than together.
acpi_backlight=intel should be tried alone first as it has less wide-ranging effects. Don't forget to run
sudo update-grub after each change made to
/etc/default/grub and then reboot to see if it has been effective.
** CW post author's note 2: my own experience (and the fact that it didn't work for OP earlier) strongly indicates that this file did not do anything good. If you are following this answer, I recommend you do not try to create this file in combination with the edit to
/etc/default/grub, unless that method did not work, and the
20-intel.conf solution did not work when tried separately.