@leocg It's a bug. I can restart, end the opera browser in task manager, update everything. install re-install. I do not know what should I do. This bug it's also on the incognito page. If you want any info just hit me up! Thank you for helping!

@leocg This Is On My Second Monitor:
Main monitor:

If I am not using a browser then instantly that white line appears but just on full screen!
The white line can appear on my main screen and also on my second screen depends what screen I am using!

I have two screens with two OperaGX tabs open. When I'm not focused on a screen or tab, a white line appears at the top of the browser. If I switch to that screen or tab, the white line moves to the other one. I added a photo also. That's the white line I am talking about!