That's not a shadow on the screen, the right hand 5th of the screen is about 20-30% dimer than the rest, messing with gamma/brightness changes the whole screen including that portion proportionally, but it's impossible to get it all matching again.
Different input has the same result, as well as no input, screen on with no input shows grey 'no input' screen with the same dark section.
It's my only monitor and a decent one at that, 2.5 years old so not happy that it's out of warranty and might not get fixed.
Thanks in advance for any help guys I appreciate your thoughts.
Also not sure why the site has turned the pic sideways, the original isn't like that.
Extra images:
https://imgur.com/xPmQ1ej
https://imgur.com/mRqvrg7
https://imgur.com/Y4uo5Ni
The entire backlight would have to be replaced, might as well just replace the monitor. I had an HP monitor for 13 years and it only got dark on the edges.