With XFCE 4.16 partially changed over to using CSD's and with Cinnamon and other desktop environments that use some Gnome Apps partially using CSD's, some users may want to make their desktop more uniform and use only the proper window manager.
PCMan's fork of gtk-mushrooms can easily do this:
Package gtk3-nocsd
sudo apt-get install -y gtk3-nocsd
Reboot. That's it; that's all there is to it.