How to fix underscan?

I've been trying to force a larger custom resolution to my mac hoping it would fix the underscan. I failed.
Whatever my resolution is, there's always that "black border" on my screen.

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On my Display settings, there is (used to be) an **Overscan** option. When checked, it's just overscanned. And when unchecked, it is underscanned. I saw some screenshots where their Display settings have an **underscan slider** instead of just a tick box.

I've tried editing my .com.apple.iokit.graphics based on this tutorial but no luck because instead of having a <key>pscn</key> in the file, mine has <key>oscn</key> which is a configuration for overscan. Its value can only be either 1 or 0, yes or no.

I've tried so much more but I still don't know how to fix it.
Here is my PC specs:
Motherboard: Gigabyte - GA-AB350-GAMING 3 ATX AM4 Motherboard
CPU: AMD - Ryzen 5 1600X 3.6 GHz 6-Core Processor
Video Card: Palit - GeForce GTX 1050 Ti 4 GB StormX Video Card

I really hope somebody can help.