That's the thing. There is no title bar. The title bar, if forced to exist, would go above both of those sections.

GNOME apps seem to have been headed in this direction for a while.

If I open gnome-disks, for example, the title bar is kind of odd because it doesn't show the name of the program at all. It only shows the size of the currently selected disk, and underneath that in a smaller text subheading is the actual device pathname of the disk. How many other programs do you know that have a subheading under the window title in the title bar?

This feels like an early decision to do something different with that part of the window.

Further along in the evolution is the dconf-editor which no longer shows any kind of title bar at all. The window manager shows that the window title is "dconf Editor" but there's nothing on the window itself that says that.

Earlier versions of each definitely had a standard title bar (I remember dconf-editor having one fairly clearly, because the new interface seemed strange at first), but not any more.

There's also that desktop web browsers generally request that their title bar not be shown. Given that everyone has at least one browser window open, it would be almost foolish to assume there's been no influence from that design choice.