<< Solved >> Shrunken desktop image on screen smaller than monitor size after 10.15.3 upgrade.

[antiflash]

After Catalina 10.15.3 upgrade the image displayed on the screen is smaller than the monitor size. Monitor resolution is 2560x1440 and is connected using DisplayPort with a Radeon 5700 XT. I have the latest version Whatevergreen with agpdmod=pikera boot option. Previous version 10.15.2 worked without problems.
Changing System Preference -> Displays -> Resolution -> Scaled to 2560x1440 shows that MacOS is displaying the full resolution (2560x1440) but it is somehow shirked to a smaller area in the center of the monitor.
Any Ideas?
Thanks in advance!

 

[antiflash]

I solved this thanks to this page

Black Borders on Your External Display? Here's the fix for macOS

If you see black borders on your external display and fonts are blurry, your overscan/underscan settings may be incorrect. It is easily fixable by going to Displays in System Preferences but the slider may be missing for monitors. Learn how to fix the issue if you don't see the slider.

ishan.co

In summary you have to edit "/var/db/.com.apple.iokit.graphics" file. It is where overscan-underscan settings are saved.
In my case one of the "pscn" keys was set to 7500, that is below the default 10000. Therfore it underscanned the monitor showing black bars as in the picture above
Just remember that in catalina to edit a system file you have to do it in recovery mode.