libadwaita updates
Another layout this leaves in the dust is tabs that are attached to a header bar but aren't full width. For example, in Nautilus. This was the original reason the tabs mockup wasn't completely implemented and kept the border btw.
Devhelp is another app with the same pattern, though it's still using GTK3 and GtkNotebook. But well, they can both switch to split header bars, all of this stuff is gonna be opt-in anyway - by using the new widgets.