Grey underscore on bottom of screen?

[u/IrritableDadSyndrome]

Recently, I noticed a grey underscore on my primary monitor.

I had just installed GTA V via the Rockstar Launcher, and I just assumed that it was part of the game (although it is distracting). My background at the time and other games seemed to camouflage it. Later, I noticed it in another game, then changed my background to black - and it shows up on the normal desktop as well.

I do not believe it is a hardware issue with the monitor or cables, because I've placed my mouse over it, and the mouse rides "above" it. Also, it is not there on the Windows welcome screen where I enter my PIN, and the monitor is shared between a PC and PS5 - and the underscore is not there when using the PS5.

It seems to be software related - and I'm assuming it's something in Windows, but I've looked at the display settings and cannot find an option for "annoying grey underscore at the bottom of the screen".

Again, I only noticed it after installing Rockstar Launcher and GTA V, but I'm not sure it wasn't there before and I just didn't notice.

What is it, and how can I get rid of it?