White line at the top of fullscreen window

Can you try restarting into Troubleshoot Mode (from Help menu) how it works without customizations enabled, and also trying out a few other themes to see if there's any difference?

Troubleshoot mode seemed to do nothing, but changing to a light theme temporarily fixed it. But any dark theme selected and the bar returns, seems to be a bug with every dark theme

Can you share your macOS version please? (It doesn't exhibit for me on 15.7.9 Sequoia trying a couple of dark themes.)

Currently on MacOS 27 Golden Gate. I think it was here in Tahoe too

Oh and is the underlying system theme light or dark?

Underlying theme is system, light or dark depending on the time of day.

Right and it reproduces using dark browser theme in both system–level light and dark appearances throughout the day the same way?

Anything specific on the system? Using a built–in display, or an external monitor, using the system resolution or setting some custom fractional scaling (i.e. "bigger text", "more content" or any other non–retina scaling), or running any accessibility accommodation features, windowing utilities, overlay apps, virtual display or video capture solutions etc.?

As it turns out, disabling increased contrast did the trick. No idea why that happened though, as I had it on in previous versions and never noticed that before.

Yeah I didn't get it even in increased contrast system UI, so wondering what all might be getting into the equation (or if the browser even can influence that on the later macOS versions to be able to work around it somehow or not).

Feel free to file a bug with your specific setup, in case somebody can reproduce and would be willing to look into some tricks how to get the system feature not interfere with that specific window edge, but it may as well end up as "wontfix" in case there's nothing to do about how the OS presents that. 🤷