Solved: getting brightness controls working on touchscreen tablet PC Panasonic FZ-G1 MkIII Toughpad

By default the controls are there but touch drag to slide them back and forth to vary brightness does not work.

Solution:

credit: Brightness Control Not working - #6 by Aravisian

A reboot is not a hindrance.

You can boot up Zorin LiveUSB, then tap the e key at the Grub menu to Edit Grub.
Find "quiet splash" on the screen and add the following parameters to it, so that it looks like (No Typos Allowed): "quiet splash i915.modeset=1 acpi_backlight=vendor acpi_osi="

Then exit the grub editor and proceed to boot normally.

If it works in that mode, then once installed, you can permanently edit grub

sudo nano /etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.modeset=1 acpi_backlight=vendor acpi_osi="

ctrl+x to exit, the y key to say yes to save, then enter key to save as current configuration.

sudo update-grub