Ask HN: Does anyone else also have a thick black line at the top of the screen?

HN does that when somebody important to tech has died.

Yes. Looks like the culprit is:

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Yes, it's an HN tradition to acknowledge the passing of notable figures in the community. On this particular occasion, the individual is Larry Tesler.

https://news.ycombinator.com/item?id=22361282

I was about to ask the same. I assumed it is because of Larry Tesler's death.