Zorin OS 16 introduced the first Zorin OS to use XFCE 4.16.
XFCE 4.16 also had changed over to using CSD's; with much drama in this decision.
Here is an XFCE specific method of restoring the Window Manager control to all windows:
sudo apt install libxfce4ui-nocsd-2-0
xfconf-query -c xsettings -p /Gtk/DialogsUseHeader -s false