ALL browsers opt-out of using System titlebars and borders, based on Chrome.
I use Chrome, Brave, and occasionally Opera, which are all descendents of Chrome … and NONE of those draw that silly border.
And Firefox opts out of that mode, and is not based on Chrome, and doesn’t draw that border.
And NONE of those browsers are in that “system border” mode, on any of my machines.
The Flatpak Chromium version doesn’t show that border … I suspect it’s a bug that was introduced recently, which has also affected the video playback - this also happened months ago.