some pixels from the background are still visible
It’s a side effect of fractional scaling. Interesting that you see it on the top like that, I only ever have it on the bottom of windows. I can’t remember if there’s something you have to enable first but starting with I think GNOME 49 you can set scaling like 133% or 166% where this is less noticeable and xwayland windows don’t look blurry, but it’s definitely still there.
So far I’ve just decided to try and ignore it, and if it really irritates you you can open up your wallpaper in GIMP and add a black bar so that it blends in.
I haven’t looked into it recently though so maybe there’s a Real Fix these days and someone will come along to this post and let us know.