Client-side decorations.
In most DEs and WMs, the compositor or the shell is responsible for the window border, Close button etc.
On GNOME, the client (application) is expected to do all of that.
There is a mechanism for a WM to request an app whether to draw CSD, and for the app to request SSD, but they don’t need to be implemented.
I don’t use Gnome3 because I dislike CSD, also Gnomes should be sending me to the Nordic mythology realm known as “the Hidden World” NOT removing features from my file manager.
What does Christopher Street Day have to do with any of this?
Client-side decorations. In most DEs and WMs, the compositor or the shell is responsible for the window border, Close button etc. On GNOME, the client (application) is expected to do all of that. There is a mechanism for a WM to request an app whether to draw CSD, and for the app to request SSD, but they don’t need to be implemented.